I built a free UK personal finance app as a single HTML file. Here's why, and how.

Jonty Collins — Mon, 23 Mar 2026 00:07:01 +0000

I'm not a developer.

A few months ago I couldn't write a line of code. I'm a British backpacker living in Australia, and about the only thing I knew about software was that I'd been frustrated by it for years.
Here's the thing — I'm a self-diagnosed personal finance geek. Spend a day with me and I guarantee I'll bring it up more times than you'd like. And yet every tool I tried to manage my money either wanted my bank login, a monthly subscription, or was built entirely around American tax law with a thin coat of paint slapped over it for UK users.

So I'd been limping along with spreadsheets. They did the job, I guess. But they never scratched the itch. I could never work out an efficient system to log my spending, so it turned into a two hour job at the end of every month. And they were completely useless for anything forward-looking — working out my CGT position, modelling what happens if I overpay my mortgage, figuring out when I could realistically stop working.

Then AI got loud enough that I couldn't ignore it anymore. I read enough to understand that the barrier between "having an idea" and "building a thing" had basically collapsed. So I set myself a challenge: what would I actually build if I could build anything?
The answer was obvious. Build the finance app I'd always wanted.

What I actually built

It's called Vault. It's a single HTML file. You open it in a browser and it works. No account. No server. No subscription. Everything you put into it lives in your browser's localStorage — which means I have zero access to any of it. Not a privacy policy promise. Structurally impossible.

It's built specifically for UK users. That meant getting the stuff right that most apps completely ignore:

Net worth tracking with debts properly deducted, multi-currency with live FX
Spending tracker with budget limits per category, recurring transactions, and spending insights
Salary calculator covering Scottish vs England/Wales/NI tax bands, pension salary sacrifice, and student loan repayments
Capital gains tax tracking using HMRC-compliant weighted average cost, with ISA holdings automatically excluded
Stamp duty calculator with the correct post-April 2025 thresholds for first-time, second home, and additional property buyers
A scenario planner covering rent vs buy, FIRE timeline, mortgage overpayment, and debt payoff
A FIRE calculator with four growth profiles
Investment (LSE) and crypto portfolio tracking

I built all of this in natural language. Genuinely. I described what I wanted, argued about the details, pushed back when something felt wrong, and iterated. Claude Code did the actual writing. Within about ten minutes of starting I had a working dark-themed web app with three sections. Claude Code had built me an entire useable website in less time than it takes to boil a pot of pasta.
I was, and still am, genuinely gobsmacked.

Why it's free and why it stays on your device

The privacy thing isn't a marketing angle. It came first.
Every app I looked at that offered the features I wanted either charged a subscription, connected to your bank and monetised that data, or both. The apps that were free were free because you were the product. Snoop makes money from bill-switching referrals using your spending data. Emma makes money from premium tiers and financial product recommendations. That's fine — they're businesses. But it means the tool is never fully on your side.

Vault has no server to send your data to. That's not a feature I added — it's just what a single HTML file is. Your financial life stays on your device. If I shut the project down tomorrow, your data doesn't disappear. You can take the file and run it locally forever.
It's free because I built it for myself first. If other people find it useful, that's the point. I'm not trying to build a business right now. I'm a backpacker who taught himself to vibe code and built the thing he wanted to exist.

Where it's at

It's live. It works. It has a first-time onboarding tour so it's not completely confusing to open cold.

It's also v1. There are things missing. There are probably bugs I haven't found yet. The data lives in localStorage which means if you clear your browser it's gone — export regularly once I build that properly.

I'm still building. I don't know exactly where it goes. But I'm sharing it now because the only way to find out if it's useful to anyone else is to let people use it.

If you try it — tell me what's broken, what's missing, what doesn't make sense. That feedback is the whole point right now.

jontycollins.github.io

Vault is built for UK users. It handles CGT, ISA tracking, UK salary calculations, and stamp duty with correct 2025/26 rates. Everything runs in your browser. No

thing leaves your device.

Written with AI assistance. Every word is based on my own experience building Vault.

DEV Community: Jonty Collins

I built a free UK personal finance app as a single HTML file. Here's why, and how.