How to Build an App Like Gusto (Payroll and HR)

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. And more than the cost, the product itself is the problem. Gusto was designed for companies running their own payroll, not for platforms selling payroll as a service to others.

This article covers what it actually takes to build a payroll and HR platform like Gusto, where the compliance traps are, and who should seriously consider it.

What a Gusto-like platform actually means to build

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. States have their equivalents. 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. The same structure applies to 401k, FSA, and HSA. 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. After the payroll is approved, net pay amounts go to a banking partner as ACH credit instructions. The bank sends the credits 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, submit time-off requests, and view their PTO balance.

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 as the input for hourly pay calculation.

Core features to build (MVP vs. full)

MVP: payroll engine and direct deposit

The minimum viable build is a working payroll engine connected to ACH direct deposit. Everything else, benefits, self-service, time tracking, can layer on top once the payroll calculation is proven correct.

Payroll engine calculation follows a specific order. Gross pay comes first: hourly workers get hours times rate, with overtime at 1.5x for hours over 40 per week (or the applicable state rule). Salaried workers get their fixed amount. Add bonuses and commissions.

Then calculate deductions in the right order. Pre-tax deductions (401k contributions, health insurance premiums, FSA contributions) come out of gross pay before taxes are calculated. Federal income tax withholding uses the IRS tax tables based on the employee's W-4 filing status. FICA takes 6.2% for Social Security (up to the annual wage base) and 1.45% for Medicare. State income tax uses each state's withholding tables. Local taxes apply in jurisdictions like New York City and Philadelphia.

Net pay is what remains after all of the above. The employer also owes its share: FICA match (6.2% + 1.45%), FUTA (6% on the first $7,000 per employee per year), and SUTA (varies by state and employer experience rating).

Every earnings, deduction, and tax record needs a permanent, immutable ledger entry. This is not a standard database table. It is an append-only financial record. You never update payroll entries. You reverse and reissue them.

Tax form generation produces the documents employees and contractors need for their personal tax filings. W-2s go to employees and the IRS each January. 1099-NECs go to contractors paid $600 or more in a calendar year. These are generated from the accumulated ledger and submitted via a tax filing API partner or payroll bureau.

ACH direct deposit requires a banking relationship. The two practical options are Stripe Treasury (Stripe's embedded finance product that provides FDIC-insured accounts and ACH access) or a direct banking partner (a bank that issues you ACH origination rights under an ODFI agreement). Stripe Treasury is the faster path for a startup. Setup takes weeks, not months. A direct banking relationship takes 3–6 months to establish and requires more compliance documentation.

Full build: benefits, self-service, and time tracking

Benefits administration tracks each employee's benefit elections and calculates the exact deduction for each pay period. An employee on the $450/month health plan with a 70/30 employer-employee split gets $135 deducted from each bi-weekly paycheck ($450 x 30% = $135, divided by 26 pay periods, with per-period rounding handled precisely).

Employee self-service adds a portal where employees can view past pay stubs, download their W-2, update their direct deposit account, request time off, and see their PTO balance. This is table stakes for any payroll product employees interact with directly.

Time and attendance adds a clock-in/out system (web, mobile, or biometric terminal), manager timesheet review and approval, automatic overtime detection based on state rules, and break compliance warnings in states that mandate rest periods.

The tech stack

The payroll engine runs on Node.js. PostgreSQL holds the employee records, pay run data, and the immutable earnings ledger. The ledger table never has UPDATE statements, only INSERTs. Every correction is a new ledger entry that reverses and replaces the prior entry.

Plaid handles bank account verification for direct deposit setup. Employees connect their bank account through Plaid's Link interface, and Plaid returns verified account and routing numbers. This prevents the single-digit error that sends a paycheck to the wrong account.

For ACH, Stripe Treasury is the fastest integration path. It provides programmatic ACH origination without requiring a direct bank relationship. Alternatively, Synapse (now Lineage) or a regional bank with ODFI status can provide ACH access.

DocuSign or HelloSign handles the onboarding forms: I-9 (employment eligibility, required for all U.S. employees within 3 days of start), W-4 (federal withholding elections), and state withholding forms. These cannot be paper-optional in a digital platform. They need an audit-ready e-signature trail.

For the payroll tax calculation itself, most teams use a third-party tax API rather than building their own tables. Symmetry Payroll Point is the standard choice. It accepts gross pay, employee location, filing status, and withholding allowances, and returns the exact amounts to withhold for federal, state, and local taxes. Smartly.io and ADP's white-label payroll bureau are alternatives. The alternative to an API is maintaining your own tax tables for all 50 states plus territories, updated every quarter. That is a dedicated compliance engineering problem, not a one-time build.

The full stack:

How long it takes and what it costs

A full payroll and HR platform runs $120,000–$200,000 over 20–24 weeks. This is longer than most software projects of similar visible scope because payroll is compliance-heavy. The 4–6 extra weeks go into tax form generation, multi-state compliance testing, and ACH error handling. None of that work is visible as a feature, but all of it is required before you can pay a single employee correctly.

An MVP covering just the payroll engine, ACH direct deposit, and basic employee records (sufficient for an EOR platform doing a closed beta) runs $75,000–$120,000 over 13–17 weeks.

The hardest technical challenge

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 of those states have cities with local income taxes. New York City, Philadelphia, and Columbus, Ohio each have their own local withholding rate that applies on top of state tax. State unemployment tax (SUTA) rates differ by state and by the employer's experience rating, which is a function of how many former employees filed for unemployment. Some states have additional local taxes, disability insurance funds, or paid family leave contributions.

Building and maintaining accurate tax tables for every jurisdiction is not a one-time engineering task. Tax rates update every January 1. Wage bases change. Some states change rates mid-year. The California SDI (State Disability Insurance) rate changes annually. The Social Security wage base changes every year. Keeping this accurate requires a dedicated compliance engineer monitoring regulatory changes across 50 states and updating the system within days of each change.

This is exactly why almost every serious payroll platform uses a tax calculation API. Symmetry Payroll Point maintains the tables. You pass in the inputs: gross wages, employee state, filing status, 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 hardest problem is ACH timing and error handling. The ACH network has strict cutoff times. To credit an employee's account on Friday, the ACH file must reach the originating bank by 8pm Eastern on Wednesday (two business days prior). Miss the cutoff and the payroll processes on Monday instead, or does not process at all. When an ACH transaction returns an error (wrong account number, account closed, insufficient funds at a receiving bank), the system needs to 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.

Build vs. Gusto: when custom wins

Gusto wins for: any company running payroll for its 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.

Custom wins for:

EOR platforms. An Employer of Record company employs workers on behalf of client companies and handles all employer obligations: payroll, taxes, benefits, compliance. Gusto's pricing is per-employee, and the EOR is processing payroll for thousands of employees across dozens of clients. The margin structure requires owning the payroll infrastructure. 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 that Gusto did not design for. The custom build handles the pay structure natively instead of forcing it through workarounds.

Vertical HR SaaS companies. A platform built specifically for restaurant groups, healthcare staffing, or construction contractors can include payroll as a native feature rather than an integration with Gusto. Owning the payroll module means controlling the product, not depending on Gusto's API stability and pricing.

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 $75,000–$120,000 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.