I replaced a £30/year tax tool with a free one — here's what I learned building it

I'm self-employed in the UK. Every year I'd pay £30+ for GoSimpleTax just to answer one question: "How much tax do I owe?"

That felt wrong. It's basically a calculator with HMRC rates plugged in. So I built my own.

TaxSimple (https://taxsimple.me) is a free UK self-assessment tax calculator. No signup, no email, instant results.

What it does:

— Income tax across all bands (20%, 40%, 45%)

— National Insurance (Class 2 + Class 4)

— All 16 allowable expense categories

— Mileage and home office deductions

— Rental income for landlords

— Payments on account

— Exact take-home pay

What I learned building it:

1. Tax logic is surprisingly tricky — the personal allowance taper above £100k alone took me 3 rewrites

2. People don't want "tax software" — they want one number: what do I owe?

3. Speed matters more than features — PageSpeed is 98/100, loads in 0.5s. Nobody wants to wait for a calculator

4. The real money isn't in the calculation — it's in the output. I charge £12.99 for an 11-page PDF report with HMRC box references. The calculation itself is free forever

5. Trust is everything in finance — I added HMRC source links, SSL badges, and a "no data stored" policy. Conversions doubled

Tech stack for the curious:

— Next.js 14 (App Router)

— Tailwind CSS

— Vercel (0.5s cold start)

— Stripe for payments

— React-PDF for report generation

What would you add next? Thinking about Scottish tax rates, a dividend calculator, or a "compare tax years" feature.

Would love feedback from anyone who files UK taxes.