How to Build a Fintech App

Decide your regulatory posture first

You are broadly choosing between three postures, and it determines your timeline more than any technical choice:

Ride someone else's licence. Operate as an agent of a sponsor bank or on a Banking-as-a-Service provider. Fastest route to launch; you inherit their compliance requirements and their oversight.

Get licensed yourself. Money transmitter licences state by state in the US, or an EMI/PI licence in the EU or UK. Months to years. Not an MVP path.

Avoid regulated activity entirely. Many fintech products never touch customer funds — analytics over bank data via Plaid, spend management on top of existing cards, reconciliation tooling. If you can stay here, your MVP ships dramatically faster.

Founders routinely assume they need the second when the first or third would serve.

KYC and AML are product surface, not paperwork

Identity verification is a user-facing flow with real drop-off, so it's a product design problem. Providers — Persona, Alloy, Middesk for businesses — handle the verification; you own where it sits in onboarding and what happens on a soft failure.

Sanctions screening against OFAC lists, transaction monitoring for velocity and structuring patterns, and the SAR-filing plumbing all need to exist even when your sponsor bank owns the filing.

The ledger mistake

Never store balances as a mutable number. The single most common and most expensive fintech engineering error is a balance column that gets updated.

Use double-entry: every movement is two immutable entries that sum to zero, and a balance is derived by summing entries. This gives you auditability, correct behaviour under concurrency, and the ability to answer "why is this number what it is" — which you will be asked, by a customer, a regulator, or your own team.

Retrofitting double-entry onto a mutable-balance system after real money has moved is among the worst migrations in software.

Choosing rails

ACH is cheap and slow with return risk for days. Wires are fast, expensive, and irreversible. RTP and FedNow are instant and irreversible. Cards are instant with chargeback exposure for months.

Irreversibility is the axis that matters: fraud strategy differs completely between a rail you can claw back and one you cannot.

Timeline

Two to three weeks for an MVP on a BaaS provider with KYC, a double-entry ledger, and one payment rail. Add the sponsor-bank approval cycle, which runs on its own clock and is usually the long pole.

Specific builds

Deeper breakdowns of products in this category:

How to Build a Payments Platform Like Stripe

FAQ

Do I need a licence to build a fintech app?

Often not for an MVP. Many products never touch customer funds — bank-data analytics via Plaid, spend management on existing cards, reconciliation tooling — and need no licence. Where you do move money, operating as an agent of a sponsor bank or on a BaaS provider is far faster than obtaining your own licence.

How should a fintech app store balances?

As derived values from a double-entry ledger, never as a mutable balance column. Every movement becomes two immutable entries summing to zero. This gives correct behaviour under concurrency, a full audit trail, and the ability to explain any figure — and retrofitting it after real money has moved is one of the worst migrations there is.

Which payment rail should an MVP use?

Choose on reversibility, since it dictates your fraud strategy. ACH is cheap and slow with multi-day return risk; wires and instant rails like RTP and FedNow are fast and irreversible; cards are instant but carry chargeback exposure for months. Most MVPs start with one rail and add others once fraud patterns are understood.