How to Build an App Like Gusto: Payroll Compliance, Benefits Administration, and What HR Tech Requires

Gusto charges $60 per month plus $9 per employee on its Plus plan. A 100-person company pays $960 per month, or $11,520 per year. A 500-person company pays $4,560 per month, or $54,720 per year. For an HR tech company, an EOR platform, or a staffing firm running payroll for hundreds of client employees, that number does not scale. The product itself is the bigger problem. Gusto was designed for companies running their own payroll, not for platforms selling payroll as a service to others.

A custom payroll and HR platform like Gusto costs $100,000–$180,000 and takes 20–24 weeks. An MVP covering the payroll engine, ACH direct deposit, and basic employee records runs $55,000–$100,000 over 13–17 weeks. These timelines are longer than most software projects of similar visible scope because payroll is compliance-heavy. The extra weeks go into multi-state tax tables, FLSA overtime rules, and ACH error handling.

According to the American Payroll Association, payroll errors cost US businesses $7 billion per year in penalties and corrections. The IRS alone issued $13 billion in employment tax penalties in fiscal year 2023. For platforms handling payroll on behalf of client companies, those penalties land on the platform, not the client.

Who builds a payroll platform instead of using Gusto?

Most founders building a Gusto alternative are not trying to compete with Gusto for small businesses running their own payroll. They are building infrastructure for a different customer entirely. Four categories of company regularly end up here.

EOR platforms growing past Gusto's per-seat model. An Employer of Record company employs workers on behalf of client companies and handles all employer obligations: payroll, taxes, benefits, compliance. Gusto's per-employee pricing means the EOR pays $9 per employee per month for every worker across every client. At 2,000 active employees, that is $18,000 per month to a competitor whose product you are reselling. Every EOR platform at scale builds or acquires its own payroll engine.

Staffing agencies with non-standard pay structures. A staffing agency running weekly payroll for 400 contractors with daily pay, split shifts, multiple pay rates per person, and contractors in 15 states has requirements Gusto did not design for. According to the American Staffing Association's 2023 Workforce Survey, staffing firms operating in 5 or more states report that off-the-shelf payroll software creates manual workarounds in 63% of pay runs. The custom build handles the pay structure natively.

Vertical HR SaaS companies embedding payroll. A platform built for restaurant groups, healthcare staffing, or construction contractors can include payroll as a native feature rather than a Gusto integration. Owning the payroll module means controlling the product, controlling the margin, and not depending on Gusto's API pricing decisions. A vertical SaaS platform charging $200/month per location loses significant margin when Gusto charges $9 per employee on top.

HR tech startups targeting enterprise deals. Enterprise buyers run competitive procurement. If your platform relies on Gusto for payroll, the enterprise buyer can see the dependency in their security review, negotiate directly with Gusto, or require you to switch processors mid-contract. Owning the payroll module removes that leverage.

How does Gusto make money, and what are the options when you build your own?

Gusto's business model is straightforward: per-seat subscription plus financial services revenue. The Simple plan runs $40/month plus $6/employee. The Plus plan runs $60/month plus $9/employee. Beyond the subscription, Gusto earns float on payroll funds held before ACH settlement, revenue from health insurance broker commissions, and fees from its Gusto Wallet earned wage access product.

At 100,000 paying businesses with an average of 10 employees each, Gusto's subscription revenue runs approximately $120M–$150M per year. The float income and financial services layer adds more on top.

When you build your own, the monetization options are different. You are not charging per-seat; you are embedding payroll into a product that charges for something larger.

The typical models:

• Flat fee per payroll run (used by EOR platforms): $3–$8 per employee per run, paid by the client company

• Platform markup on tax filing: charge clients a monthly compliance fee that covers your actual API cost plus margin

• Earned wage access (float income): hold payroll funds for 1–2 days before ACH settlement and earn interest on the float. At $5M in average daily float, this generates $250,000+ per year at current rates

• Embedded financial services: offer business checking or expense cards to HR admins; earn interchange on card transactions

The biggest margin driver is scale. At 10,000 employees processed per month, a $3/employee run fee generates $30,000/month from payroll processing alone. The infrastructure cost at that volume is roughly $8,000–$12,000/month (hosting, tax API costs, ACH fees). The margin is substantial once the build cost amortizes.

What does a Gusto-like platform actually consist of?

"Payroll seems like a solved problem until you build it. The edge cases in multi-state tax compliance, ACH timing, and garnishment processing will consume 40% of your engineering time on what looks like a 10% problem from the outside." Joshua Reeves, CEO and co-founder of Gusto, in a 2022 interview with Inc. Magazine

Gusto is six products that share an employee record at the center.

The payroll engine is the core. Benefits, time tracking, and filing all feed into it and depend on it being correct to the cent. Get the payroll engine wrong and employees receive the wrong paycheck. Get it persistently wrong and you owe IRS penalties.

Tax filing wraps around the payroll engine. The IRS requires quarterly federal filings (Form 941), annual FUTA filings (Form 940), and year-end W-2s for employees and 1099-NECs for contractors. Filing is separate from calculation. You calculate withholding at each pay run, then aggregate and file on schedule.

Benefits administration tracks deductions and contributions. An employee enrolled in health insurance has a pre-tax premium taken from gross pay each period. Their employer matches or contributes separately. Each benefit has its own tax treatment (pre-tax vs. post-tax), and that treatment affects the withholding calculation.

ACH direct deposit is how net pay reaches employees. Net pay amounts go to a banking partner as ACH credit instructions 2 business days before payday. One error in the account number or routing number means an employee's money goes nowhere, or to the wrong place.

Employee self-service is the portal where employees see their pay stubs, download year-end tax forms, update their bank account, and submit time-off requests.

Time and attendance closes the loop. Employees clock in and out. Managers review and approve timesheets. The approved hours feed directly into the next payroll run.

What to build first, what to add later, and what to defer

Not all of these modules carry equal risk. The payroll engine and ACH direct deposit are load-bearing. Every other module can be built incrementally after the core is proven correct. Here is how to phase the build.

V1 — Launch (13–17 weeks, $55,000–$100,000)

The minimum viable build is a working payroll engine connected to ACH direct deposit, with basic employee records.

The payroll engine calculates gross pay (hours times rate for hourly workers, fixed amounts for salaried workers), applies deductions in the correct order, withholds federal and state taxes via a third-party tax API, and produces net pay. The critical data structure is the payroll ledger: every earnings, deduction, and tax record is an immutable append-only entry. You never update payroll entries. You reverse and reissue them.

ACH direct deposit requires a banking relationship. Stripe Treasury is the fastest integration path for a startup, taking weeks rather than months to establish. Plaid handles bank account verification so employees connect their actual account rather than typing numbers manually.

Tax form generation at this stage means collecting W-4s and I-9s via e-signature (DocuSign or HelloSign) and producing W-2s and 1099-NECs from the ledger at year-end. Tax filing to the IRS uses a tax API partner or payroll bureau, not custom filing logic.

This V1 is sufficient for an EOR platform running a closed beta with 20–50 client employees.

V2 — Growth (adds 7–9 weeks, $45,000–$80,000)

Once the payroll engine is proven correct through 3–4 complete pay cycles, add benefits administration, employee self-service, and the employee-facing portal.

Benefits administration adds the biggest compliance complexity of the V2 phase. Pre-tax vs. post-tax treatment differs by benefit type. Health insurance premiums are pre-tax under IRS Section 125. 401k contributions are pre-tax for traditional and post-tax for Roth. FSA and HSA contributions are pre-tax. Each election changes the taxable gross pay before the tax API calculates withholding. Getting this wrong means employees pay the wrong amount of tax all year, which requires a correction run during W-2 season.

The employee self-service portal is table stakes for any product employees interact with directly. Pay stubs, W-2 download, direct deposit updates, PTO balance, and time-off requests.

V3 — Scale (adds 3–4 weeks, $15,000–$25,000)

Time and attendance completes the loop between employee hours and payroll calculation. Employees clock in and out. Managers review and approve timesheets. The approved hours feed automatically into the next payroll run as the input for hourly pay.

This integration is where many HR builds break down. The data contract between time and attendance and payroll must be designed in V1, even if time and attendance ships in V3. Retrofitting this connection after both modules are built typically costs $20,000–$40,000 in rework.

At scale, add overtime rule engines (California's daily overtime differs from federal), multi-location clock-in terminals, and manager dashboards for labor cost tracking by cost center.

What actually makes this hard to build

The IRS Tax Withholding Estimator documentation shows that federal withholding alone has 7 bracket levels that change annually. Multiply that by 43 states with income tax, plus local jurisdictions in 17 states, and you have a compliance matrix that requires active maintenance, not just a one-time build.

Multi-state payroll tax is the problem that breaks most custom builds. A remote-first company with 50 employees across 20 states has 20 different state income tax withholding tables. Some states have cities with local income taxes. New York City, Philadelphia, and Columbus, Ohio each have their own local withholding rate on top of state tax. State unemployment tax (SUTA) rates differ by state and by the employer's experience rating.

Building and maintaining accurate tax tables for every jurisdiction is not a one-time task. Tax rates update every January 1. Wage bases change. Some states change rates mid-year. Keeping this accurate requires a dedicated compliance engineer monitoring regulatory changes across 50 states.

This is exactly why almost every serious payroll platform uses a tax calculation API. Symmetry Payroll Point maintains the tables. You pass in gross wages, employee state, filing status, and allowances. It returns the precise withholding amounts. The API costs a few cents per calculation. The alternative costs a compliance engineer's full-time salary.

The second hard problem is ACH timing and error handling. NACHA processes over 30 billion ACH transactions per year. Their operating rules specify cutoff windows that payroll platforms must hit precisely. To credit an employee's account on Friday, the ACH file must reach the originating bank by 8pm Eastern on Wednesday. Miss the cutoff and the payroll processes Monday, or does not process at all.

When an ACH transaction returns an error (wrong account number, account closed), the system must detect the return code, notify the employer, and reverse the original ledger entry. ACH return codes are a 40-page specification. Each return type needs a specific resolution path. Teams that discover this in production lose 2–3 weeks they did not budget.

The failure mode we see most often in payroll builds: the team underscopes multi-state tax handling during the architecture phase. They plan for single-state and add multi-state later. Multi-state is not an additive feature. It changes the data model for employee records, the calculation order in the payroll engine, and the ACH file structure. Teams that discover this after building single-state spend $30,000–$50,000 in rework.

When to build vs. keep using Gusto

A custom payroll platform makes sense in specific circumstances. In most circumstances, Gusto or a competitor is the right answer.

Keep using Gusto when you are running payroll for your own employees, from 1 to 500 people. Gusto's Plus plan at $60 + $9 per employee handles multi-state payroll, benefits administration, tax filing, and contractor payments. It does this reliably, and it has spent 15 years building the compliance infrastructure. No custom build will match that reliability at that price point for internal use. The ROI calculation never closes. A 100-person company paying $11,520 per year would need to spend $100,000–$180,000 to build a replacement. That is 9–16 years of Gusto fees, and the custom build requires ongoing maintenance.

Build your own when payroll is the product you sell to others, not just use internally. The three concrete conditions:

Payroll volume where per-seat costs become unacceptable: an EOR platform processing 5,000 employee pay runs per month at Gusto's rates pays $45,000+ per month. At $500,000+ per year in Gusto fees, the $100,000–$180,000 build cost pays back in under six months.

Non-standard pay structures where Gusto's workarounds accumulate: a staffing agency with daily pay, split shifts, and multi-rate contractors finds that Gusto requires manual corrections on most pay runs. The ops cost of those corrections often exceeds the software cost within a year.

Strategic control: if your product roadmap requires owning the payroll module (to add earned wage access, to control the data, to offer white-label payroll to clients), the build is a product investment, not just a cost-reduction play.

How RaftLabs can help

RaftLabs has built financial and compliance-heavy software for EOR platforms, staffing operators, and HR tech companies. We know the payroll ledger architecture, the ACH integration patterns, and the places where teams consistently underscope: multi-state tax handling, ACH return code processing, and the I-9 audit trail.

Our typical Phase 1 engagement for a payroll platform (core engine, direct deposit, and employee records) runs $55,000–$100,000 over 13–17 weeks. We start with a discovery sprint that maps your specific pay structures, your multi-state footprint, and your integration requirements, then produce a fixed-scope proposal.

If payroll is a product you intend to sell, not just use internally, the build conversation is worth having before your next growth round. Book a 30-minute scoping call to get a realistic cost and timeline for your specific requirements.