How to Fill Out the PSD Air Construction Permit Application + FAQs

Filing a Prevention of Significant Deterioration (PSD) air construction permit application is one of the most demanding tasks in environmental compliance. It is a federal preconstruction permit required before you can build a new major source of air pollution—or significantly modify an existing one—in an area that meets national air quality standards. Get it right, and your project moves forward on schedule. Get it wrong, and you can face months of delays, costly redesigns, or even an enforcement action for building without authorization.

This guide walks you through the entire application, line by line, the way someone who has filed dozens of these would explain it across the kitchen table. We will cover what the form is, who must file, the documents you need, how to fill out every major section, three realistic filled-out examples, how to submit, what happens next, the mistakes that sink applications, and a full FAQ. The rules below are anchored in the federal PSD regulations at 40 CFR 52.21 and the federal permitting procedures at 40 CFR Part 124, Subpart C.

One important note before you start. There is no single nationwide PSD form with a fixed revision date. PSD is implemented either directly by the U.S. EPA or by a delegated state, local, or tribal agency, and each agency packages its application differently. Because of that, this article treats the PSD application as a standard application package—the universal set of forms, calculations, and analyses every authority requires—rather than one numbered PDF. Where your agency uses its own numbered forms (for example, Wisconsin’s 4530 series or Washington’s PSD Applicability Determination form), the line-by-line logic in this guide maps directly onto those forms.

What the PSD Permit Application Is and Who Must File It

The PSD permit is a preconstruction air permit. The word “preconstruction” is the heart of it: you must hold the permit before you break ground, pour a foundation, or order major equipment for the regulated activity. Construction that begins before issuance can void your application and trigger penalties. The program exists to keep clean-air regions—technically called attainment or unclassifiable areas under the National Ambient Air Quality Standards (NAAQS)—from slowly degrading as new industry arrives.

You must file a PSD application if your project is a new major stationary source or a major modification of an existing major source in an attainment area. A “major source” is defined by its potential to emit (PTE). Under 40 CFR 52.21, the threshold is 250 tons per year (tpy) of any regulated NSR pollutant for most facilities. That drops to 100 tpy if your facility belongs to one of the 28 listed source categories—things like fossil-fuel power plants, petroleum refineries, cement plants, and steel mills, as summarized by both South Carolina DES and a clear overview from Stevens EHS.

For an existing major source, a “major modification” is a physical or operational change that causes a significant net emissions increase of any regulated pollutant. The significance trigger is small relative to the major-source threshold—often as little as 40 tpy of volatile organic compounds (VOC) or nitrogen oxides (NOx), as explained in this Williams Mullen analysis. This is why even seemingly modest projects at large plants can pull you into full PSD review.

If you are subject, your completed application must demonstrate four things, as the EPA frames the program: installation of the Best Available Control Technology (BACT), a satisfactory air quality analysis, an additional impacts analysis, and participation in a public involvement process. Every section of the application you fill out feeds one of those four pillars.

Before You Start: Documents and Information You Need

PSD applications fail more often from missing back-up data than from a poorly worded narrative. Assemble everything below before you type a single number into the form. Treat this as your master checklist, modeled on the comprehensive Wisconsin DNR PSD Application Checklist.

• Facility identification details: legal owner name, operator name, mailing address, physical site address, latitude and longitude, and your Standard Industrial Classification (SIC) or NAICS code.
• A scaled plot plan showing property boundaries, the fence line or “ambient air” boundary, building footprints with heights, and the location and height of every emission stack.
• Process flow diagrams for each new or modified process, showing where emissions are generated and where control devices sit.
• Equipment specifications: rated capacities, fuel types, heat input ratings, hours of operation, and manufacturer emission data.
• Emission calculations for every pollutant, with the emission factors clearly cited (AP-42 factors, vendor guarantees, stack-test data, or mass balance).
• Five years of contemporaneous emissions records if you are modifying an existing source, because increases and decreases over that window affect your netting analysis, a point stressed in the Stevens EHS overview.
• A full top-down BACT analysis for each pollutant that exceeds its significant emission rate.
• Air dispersion modeling inputs and electronic model files, typically built in AERMOD, EPA’s preferred near-field model.
• A list of nearby Class I areas (national parks and wilderness areas) and the Federal Land Managers who oversee them.

Gather permits and prior applications too. If your site already holds a Title V operating permit or earlier construction permits, the permit numbers, conditions, and any synthetic minor limits must be reflected accurately so your baseline emissions are not double-counted. Missing or stale baseline data is one of the most common reasons reviewers issue a completeness deficiency letter.

Where to Get the Form and How to Access It

Your first decision is identifying the correct permitting authority, because that determines which application package you download. PSD authority falls into one of three buckets: EPA implements the program directly in some areas; a state or local agency runs it under a SIP-approved or delegated program; or a tribe administers it. The EPA maintains region-specific delegation tables, such as the Region 9 delegation and PSD SIP status page, which tell you exactly who issues your permit.

Once you know the authority, get the forms from that agency’s air-permitting webpage. Most agencies publish a downloadable PSD application package or a set of fillable PDF and spreadsheet forms. For example, Minnesota PCA offers both an e-Services online submission and downloadable paper forms, while Wisconsin DNR provides a numbered series of forms covering facility ID, plot plan, stack identification, and emission summaries.

Filing channels vary, and you should confirm yours before assembling copies. The most common channels are an online portal, email submission, mailed paper copies, or in-person delivery. Some agencies still require two complete paper copies plus a PDF on a USB drive, as Minnesota PCA notes for non-electronic filings. When in doubt, default to the agency’s online portal if one exists, because portals timestamp your submission and reduce the risk of a lost mailing.

Schedule a pre-application meeting before you finalize anything. Both Washington Ecology and most other authorities strongly encourage this step. You can bring a partially completed applicability determination, walk the reviewer through your project, and lock in the modeling protocol—saving weeks of rework later.

Step-by-Step: How to Fill Out the PSD Application Line by Line

This is the core of the application. Work through each section in order, because later sections depend on numbers you establish early. Every subsection below corresponds to a logical grouping found in virtually every agency’s PSD package.

Section 1: Facility and Owner Identification

Start with the administrative block. Enter the legal name of the company that owns the source exactly as it appears on your corporate registration, not a trade name or abbreviation. Mismatched names between the application and your existing permits are a frequent and avoidable deficiency.

Next, enter the operator if it differs from the owner, then the physical location of the facility. The physical address must be the site where construction occurs, not your corporate headquarters. Provide latitude and longitude to at least five decimal places, because the modeling and Class I area screening depend on precise coordinates.

Enter your primary SIC and NAICS codes. These codes matter beyond bookkeeping: they determine whether your 100-tpy or 250-tpy major-source threshold applies, since the 28 listed categories in 40 CFR 52.21 are tied to industry type. Finally, list every existing air permit number for the site so the reviewer can connect your application to your regulatory history. Take your time here; a clean identification block signals a careful applicant and sets a good tone for the entire review.

Section 2: Project Description Narrative

The narrative is where you tell the story of the project in plain language. Describe what you are building or changing, why, and how it ties into the rest of the facility. The Wisconsin DNR checklist specifically asks for a detailed description of the project, how it integrates with existing operations, and its effects on production, economics, and the environment.

Be concrete. State the equipment being added, the rated capacities, the fuels, and the expected operating schedule in hours per day and days per year. If you intend to accept operating limits to stay below a threshold, say so explicitly, because those limits will become enforceable permit conditions.

Avoid marketing language and avoid vagueness. Reviewers read the narrative first and use it as a map for everything that follows. A clear narrative that matches your emission calculations and plot plan builds credibility; contradictions between the narrative and the data invite scrutiny and questions. Write it so that a reviewer who knows nothing about your plant can picture the project after one read.

Section 3: Emission Units, Stacks, and Plot Plan

Here you inventory the physical hardware. List every new or modified emission unit with a unique identifier, its capacity, and its associated control device. Then describe each stack: height, inside diameter, exit gas temperature, and exit velocity at both normal and maximum operating conditions. These stack parameters are not busywork—they are the direct inputs to your air dispersion model, as the Wisconsin checklist makes clear.

Attach a scaled plot plan. It must show property boundaries, the ambient air boundary (the line beyond which the public can access), building footprints with their heights for downwash analysis, and the precise location of each stack. Include a larger-scale map showing nearby streets, terrain, and other emission sources.

Accuracy in this section pays off twice. First, the modeler relies on it directly. Second, the reviewer cross-checks your stacks against your emission units and your narrative. A stack listed in the modeling but missing from this inventory is the kind of inconsistency that generates a deficiency letter, so reconcile all three documents before submitting.

Section 4: Potential to Emit and Applicability Determination

This section answers the threshold question: is your project actually subject to PSD? Calculate the potential to emit (PTE) for every regulated NSR pollutant. PTE is the maximum emissions a source can produce running at full design capacity, adjusted only for federally enforceable limits. Compare your facility-wide PTE against the 100-tpy or 250-tpy major-source threshold, using the approach laid out by Stevens EHS.

For an existing major source, run the netting analysis instead. Calculate baseline actual emissions, calculate projected future emissions, and find the net increase for each pollutant. Then fold in contemporaneous increases and decreases over the prior five-year window, because those can change the result, as the Stevens EHS overview explains and as Minnesota’s WS-01 worksheet is designed to document.

Compare each net increase to its significant emission rate (SER). The SERs are defined in 40 CFR 52.21 and are pollutant-specific. The table below shows commonly cited values; always confirm against the current rule because they are periodically revised.

Pollutant	Significant Emission Rate (tpy)
Carbon monoxide (CO)	100
Nitrogen oxides (NOx)	40
Sulfur dioxide (SO2)	40
PM2.5	10
Volatile organic compounds (VOC)	40
Lead	0.6

Document every assumption. Show your emission factors, cite their source, and state plainly why you did or did not include support facilities and units that may be debottlenecked. The applicability determination is the legal foundation of the entire application, so over-document rather than under-document here.

Section 5: Best Available Control Technology (BACT) Analysis

For every pollutant whose net increase exceeds its SER, you must perform a top-down BACT analysis. There is usually no dedicated form for this; you attach it as a standalone document, as the Wisconsin DNR checklist notes. The five-step top-down method is the universally accepted structure, summarized cleanly by Nebraska DEE and described in detail by SCAQMD.

1. Identify all available control options, including add-on controls, lower-emitting processes, and work practices, and cite the source for each.
2. Eliminate technically infeasible options, demonstrating infeasibility on physical, chemical, or engineering grounds—cost may not be used at this step.
3. Rank the remaining technologies from most to least effective, listing percent removal and the resulting emission rate.
4. Evaluate the energy, environmental, and economic impacts of the top options, working down the ranked list until you reach a defensible choice.
5. Select BACT, proposing a specific emission limit, an averaging time, and a compliance demonstration method.

The single most contested part of any PSD review lives in Step 4. Reviewers and the EPA Environmental Appeals Board have remanded permits when applicants dismissed the top-ranked control on weak economic grounds. If you reject the most effective technology, your cost-effectiveness numbers (dollars per ton removed) must be thorough and well-sourced. Write each step as its own clearly labeled subsection, and prepare a separate BACT analysis for each pollutant rather than blending them, because reviewers evaluate them separately.

Section 6: Air Quality Analysis (Dispersion Modeling)

Every PSD permit requires an air quality analysis demonstrating that your project will not cause or contribute to a violation of the NAAQS or consume more than the allowable PSD increment, as both EPA and Iowa DNR describe. The increment is the small slice of additional pollution allowed above a baseline concentration in a clean-air area.

Build the analysis in AERMOD, EPA’s preferred near-field model, which accounts for terrain, building downwash, and refined meteorology. Before you run anything, lock down a modeling protocol with your agency—the inputs, meteorological data set, receptor grid, and background concentrations. Submit the protocol, any EPA correspondence, and the electronic model files with your application, as Florida DEP best practices recommend.

Provide the stack parameters and emission rates that feed the model, along with the scaled plot plan and the larger-area map. Present both a significant-impact analysis (does your project even rise to a level requiring full modeling?) and, if it does, a full cumulative analysis adding nearby sources and background. Submit the electronic files in the format the agency requests; a modeling demonstration that the reviewer cannot reproduce will be rejected regardless of its conclusions.

Section 7: Additional Impacts Analysis

Beyond the NAAQS and increment, PSD requires a separate look at collateral effects. This additional impacts analysis covers three areas, as itemized in the Wisconsin checklist: a growth analysis estimating the secondary residential and commercial growth your project will induce; a soils and vegetation analysis examining effects on local agriculture and plant life; and a visibility analysis for nearby Class II areas.

The growth analysis often surprises first-time filers. You are not just modeling stacks; you are estimating how many workers your project draws to the area and the associated emissions from new housing, traffic, and services. Keep the estimate realistic and tie it to your construction and operating workforce numbers.

For soils, vegetation, and visibility, you may often rely on the fact that meeting the NAAQS protects most vegetation and soils, but you must still document that conclusion rather than assume it. Where sensitive crops, specialty vegetation, or scenic vistas sit near your fence line, expand this section accordingly. Reviewers want to see that you considered these impacts deliberately, not that you skipped them.

Section 8: Class I Area Impact Analysis and Federal Land Manager Notification

If a Class I area—a national park or wilderness area with special air protections—lies within range of your project, you must analyze your impact on it. This includes a Class I increment analysis and an Air Quality Related Values (AQRV) analysis covering visibility, sensitive ecosystems, and deposition, as the National Park Service describes for its review role.

Notification of the Federal Land Manager (FLM) is mandatory and time-sensitive. The permitting authority must notify the relevant FLM, and you should provide your application materials to them. Under EPA’s procedures, FLM notification is given within 30 days of receipt and at least 60 days before any public hearing, as the Region 9 FLM notification page explains. Use your agency’s Class I area locator, like the one referenced in Florida DEP’s guidance, to confirm whether any Class I area is in play.

Do not treat this section as optional just because the nearest park seems far away. AQRV impacts, especially on visibility and nitrogen deposition, can reach surprising distances. If you are unsure, raise it at your pre-application meeting and let the reviewer and FLM tell you the screening radius they expect.

Section 9: Certification and Signature

The application closes with a certification signed by a responsible official—typically a corporate officer or a designated environmental manager with authority to bind the company. The signature certifies, under penalty of law, that the information is true, accurate, and complete. Do not let a junior staffer sign; an improper signatory can render the application incomplete.

Read the certification language before signing. It carries real legal weight, and knowingly submitting false information exposes the signatory to civil and potentially criminal liability. Make sure every number on the signed application matches the back-up calculations in your attachments.

Finally, assemble the package in the order the agency specifies, label every attachment, and keep a complete copy for your records. A well-organized, paginated submission is processed faster than a loose pile of documents, and it demonstrates the diligence reviewers reward.

Three Filled-Out Examples Using Real Scenarios

Concrete examples make the abstract rules click. Here are three realistic fact patterns showing how the same application behaves under different circumstances.

Example 1: New Natural Gas Combustion Turbine Power Plant

A developer proposes a new 200-megawatt natural gas combined-cycle power plant in a rural attainment county. Because fossil-fuel-fired steam electric plants are one of the 28 listed source categories, the major-source threshold is 100 tpy, per 40 CFR 52.21. The facility’s PTE for NOx is calculated at roughly 180 tpy, which clearly makes it a major source and triggers full PSD review.

In Section 5, the BACT analysis for NOx identifies selective catalytic reduction (SCR) combined with dry low-NOx burners as the top-ranked control. The applicant proposes SCR with a permit limit of 2.0 ppm NOx and continuous emissions monitoring as the compliance method. The CO and VOC analyses propose an oxidation catalyst. In Section 6, AERMOD modeling shows the project’s maximum NOx impact stays below the significant impact level, simplifying the cumulative analysis. The takeaway: for a clean fuel like natural gas, the heavy lifting is the BACT cost analysis, not the modeling.

Example 2: Major Modification at an Existing Petroleum Refinery

A refinery already operating as a major source plans to add a new heater to expand throughput. Refineries are a listed category, so significance—not the major-source threshold—governs whether the change is a major modification. The applicant runs the netting analysis in Section 4 using a worksheet like Minnesota’s WS-01, calculating baseline actual emissions and projected actual emissions for the new heater.

The net NOx increase comes to 55 tpy, above the 40-tpy SER, so the modification is major for NOx. Crucially, the applicant must also fold in a contemporaneous 20-tpy NOx decrease from a burner replacement completed two years earlier, which still leaves a net increase above the SER. Sulfur dioxide, by contrast, nets out at 30 tpy—below its 40-tpy SER—so SO2 escapes BACT. This example shows why the five-year contemporaneous window and pollutant-by-pollutant comparison, both stressed by Stevens EHS, can make or break which pollutants drive your review.

Example 3: New Cement Plant Near a Class I Wilderness Area

A company proposes a new Portland cement plant—another listed 100-tpy category—located 40 kilometers from a designated Class I wilderness area. PTE for PM2.5, NOx, and SO2 all far exceed their thresholds, so the project is unambiguously major and requires BACT for several pollutants. The PM BACT analysis proposes a fabric filter baghouse with a tight grain-loading limit.

Because of the nearby Class I area, Section 8 becomes the critical path. The applicant performs a Class I increment analysis and an AQRV analysis for visibility and nitrogen deposition, and the permitting authority notifies the Federal Land Manager within the timelines on EPA’s FLM notification page. The FLM raises a visibility concern, prompting the applicant to tighten the proposed NOx and SO2 limits. The lesson: when a Class I area is in range, build extra months into your schedule for FLM coordination.

How to File the Completed Form

With the package complete, file through the channel your authority uses. If an online portal exists, it is usually the best choice because it timestamps receipt and confirms which documents arrived. The Bay Area AQMD, for example, lets applicants apply online or download paper forms to submit by email or mail.

If you are filing on paper, follow the copy requirements exactly. Some agencies require two complete paper copies plus an electronic PDF on a USB drive, as Minnesota PCA specifies for non-electronic submissions. Always include your electronic modeling files in the format the agency requested; a missing model file alone can stall a completeness review.

Pay any required application fee at submission. Fees are jurisdiction-specific and can range from modest filing charges to tens of thousands of dollars for complex major-source reviews, so confirm the current amount with your authority before filing. A check or portal payment that does not match the published fee schedule can delay the start of your review clock.

Keep proof of submission. Save the portal confirmation, certified-mail receipt, or email acknowledgment, and retain a full copy of everything you sent. If a dispute later arises about what was submitted or when, that record is your protection. Filing is not the finish line, but a clean, complete, well-documented submission is the single best predictor of a smooth review.

What Happens After You File

Once received, the agency begins a completeness review. They check that every required form, calculation, and analysis is present before they start the substantive technical review. If something is missing, they issue a deficiency or completeness letter, and your review clock effectively pauses until you respond. Prompt, organized responses keep your project moving.

After the application is deemed complete, the agency conducts its technical review, scrutinizing your applicability determination, BACT selection, and modeling. For PSD applications, the authority then prepares a draft permit and opens a public comment period, as required under 40 CFR Part 124. Public comment periods commonly run 30 days, and the agency must respond to significant comments before issuing a final permit. Federal Land Managers are notified within 30 days of receipt and at least 60 days before any hearing when a Class I area is involved, per EPA Region 9.

Plan for a long timeline. A typical PSD review takes roughly a year from a complete application to a final permit, and complex projects with Class I issues or contested BACT can take longer. Only after the final permit issues—and any appeal window closes—may you begin construction of the regulated activity. Building before then is precisely what the program forbids.

Mistakes to Avoid When Filling Out the Form

The same errors sink applications year after year. Knowing them in advance is the cheapest insurance you can buy.

• Starting construction before the permit issues. PSD is a preconstruction permit; early groundwork can void your application and trigger enforcement.
• Underestimating potential to emit by using realistic operating hours instead of full design capacity, unless you have accepted federally enforceable limits.
• Forgetting contemporaneous emissions in a netting analysis, which can flip a project from minor to major, as Stevens EHS warns.
• Using cost to eliminate a control option in Step 2 of the BACT analysis, where only technical infeasibility is allowed, per the Wisconsin checklist.
• Dismissing the top-ranked BACT control with thin economic justification, a frequent cause of permit remands by the EPA Appeals Board.
• Running dispersion modeling without an agency-approved protocol, then having to redo it.
• Omitting the electronic modeling files, which makes your analysis unreproducible and incomplete.
• Inconsistencies among the narrative, the emission unit inventory, the plot plan, and the modeling inputs.

The thread connecting these mistakes is internal inconsistency and missing documentation. Reviewers are not trying to trip you up; they simply cannot approve numbers they cannot trace. Before you submit, perform a self-audit: pick three random emission values and follow each one from the summary table back to its underlying calculation and cited emission factor. If any number breaks the chain, fix it before the agency finds it.

Do’s and Don’ts

A quick reference for the habits that separate clean applications from troubled ones.

Do:

• Schedule a pre-application meeting and lock down your modeling protocol early, as Washington Ecology recommends.
• Calculate PTE at full design capacity unless you accept enforceable limits.
• Document the source of every emission factor and assumption.
• Prepare a separate, fully labeled top-down BACT analysis for each pollutant.
• Cross-check the narrative, inventory, plot plan, and modeling for consistency.
• Keep proof of submission and a complete copy of the package.

Don’t:

• Don’t begin any regulated construction before the final permit issues.
• Don’t use cost to eliminate technologies at BACT Step 2.
• Don’t ignore contemporaneous increases and decreases in netting.
• Don’t submit modeling without the electronic files.
• Don’t let an unauthorized person sign the certification.
• Don’t assume a distant Class I area is irrelevant without screening it.

Following these habits will not guarantee a fast permit, because PSD review is inherently thorough. They will, however, eliminate the self-inflicted delays that cost applicants the most time.

Pros and Cons of Filing on Your Own vs. With Help

Most major-source applicants hire an environmental consultant or air-quality engineer, but smaller modifications are sometimes handled in-house. Here is how the trade-off looks.

Approach	Pros	Cons
Filing on your own	Lower upfront cost; deep internal knowledge of your own process; full control of the schedule	Steep technical demands for BACT and AERMOD modeling; high risk of completeness deficiencies; staff time diverted from operations
Filing with a consultant	Modeling and BACT expertise; familiarity with agency expectations and protocols; fewer deficiency letters and faster review	Professional fees that can be substantial; need to transfer detailed process knowledge to the consultant; less direct control

The honest middle ground is a hybrid. Many companies keep the project narrative and emission calculations in-house, where process knowledge lives, and bring in a specialist for the dispersion modeling and the defensibility of the BACT cost analysis. The BBJ Group’s guidance on air permit applications underscores that starting from the local agency’s website and forms is essential regardless of who does the work.

Decide based on the project’s complexity and your internal capacity. A single boiler replacement with simple netting may be manageable in-house. A new power plant or any project touching a Class I area almost always warrants experienced help, because the modeling and FLM coordination are unforgiving of error.

FAQs

What does PSD stand for, and why does it matter? PSD stands for Prevention of Significant Deterioration. It is a federal preconstruction permitting program under the Clean Air Act that keeps clean-air regions from degrading, as EPA explains. It matters because you generally cannot build a major source or major modification in an attainment area without one.

How do I know if my project triggers PSD review? Calculate your potential to emit and compare it to the major-source threshold of 100 or 250 tpy depending on your industry category, then, for existing sources, run a netting analysis against pollutant-specific significant emission rates, following the method described by Stevens EHS. If any pollutant exceeds its trigger, you are likely subject to PSD.

Who issues the PSD permit—EPA or my state? It depends on where you are. EPA issues permits directly in some areas, while many states, local agencies, and tribes run delegated or SIP-approved programs, as shown in EPA’s delegation status tables. Confirm your authority before downloading any forms.

What is BACT, and how is it determined? BACT is the Best Available Control Technology, determined case by case using the five-step top-down method described by Nebraska DEE. You identify all controls, eliminate infeasible ones, rank the rest, evaluate impacts, and select a control with an enforceable emission limit.

How long does the PSD permit process take? Plan for roughly a year from a complete application to a final permit, including the public comment period required under 40 CFR Part 124. Complex projects, especially those affecting Class I areas, can take longer.

Can I start construction while my application is pending? No. PSD is a preconstruction permit, meaning you must hold it before beginning the regulated construction, a point emphasized by Washington Ecology. Early construction can void your application and trigger penalties.

What air dispersion model should I use? AERMOD is EPA’s preferred near-field model for most PSD applications because it handles terrain and building downwash, as described by Stevens EHS. Always confirm the model and protocol with your agency before running it.

What is a Class I area, and how does it affect my application? Class I areas are national parks and wilderness areas with heightened air protections. If one is within range, you must perform a Class I increment and AQRV analysis and coordinate with the Federal Land Manager, whose notification timelines EPA sets out on its FLM notification page.

Do I really need a consultant? Not always, but for complex projects it is strongly advised. The modeling and BACT cost analysis are technically demanding, and starting from your agency’s own forms and guidance, as BBJ Group recommends, is essential whether you file alone or with help.