Tax Tracker

The problem

I run a limited company outside IR35, and the recurring question is dull but expensive to get wrong: how much of what is sitting in the business account is actually mine, and how much do I need to put aside for tax. Spreadsheets answer that until they do not, and most off-the-shelf tools are built for accountants rather than the contractor doing their own books between client work. So I built the thing I wanted and have used it on my own finances since.

What I built

A full-stack web app with a companion Expo / React Native app sharing the same auth and data layer. The parts that earned their place:

• Tax estimates that match my real situation. Deterministic calculation of VAT, Corporation Tax and Income Tax, plus a working-day calculator and director’s loan account (DLA) tracking with an s455 estimate. The numbers are computed in code, not by a model, so I can defend every figure.
• Receipt OCR. Photograph a receipt and a Gemini multimodal pass returns structured expense data (supplier, date, totals, VAT, category) with field-level confidence and a graceful fallback when it cannot read something.
• Bank reconciliation. Imported transactions are matched against recorded entries by a Gemini matcher that proposes confidence-scored accounting actions. Account numbers, sort codes, card and IBAN details are redacted before anything reaches the model, and high-confidence matches apply automatically while the rest wait for review.
• Year-end handover. A Google Sheets export so my accountant gets clean figures without a back-and-forth.

The hard part

The tax was the hard part, less the individual formulas than the way the figures depend on each other. I take a low director’s salary plus dividends. The dividend I can actually draw depends on retained profit after Corporation Tax, and the salary itself feeds Corporation Tax, Income Tax and National Insurance, so no single number sits still long enough to be computed on its own. On top of that sit the VAT scheme, the dividend bands and allowance, and the director’s loan account: an overdrawn loan not cleared within nine months and a day of the company year-end triggers an s455 charge, reclaimable once repaid. That clock runs on the company accounting period, not the 6 April personal tax year, so I track two calendars separately. Rates and thresholds move every year, so the whole thing is parameterised by tax year rather than hard-coded, and it models my own company, not every company in the country. A wrong figure here is expensive and has to stand up to HMRC and to my accountant, so the calculations stay in deterministic code and the model never computes one. Gemini only ever proposes bookkeeping, receipt fields and reconciliation matches, each carrying a confidence score, and every run writes an audit row so I can see why a suggestion was made.

Status

In daily use for my own company. Built solo, spec-first: I direct an AI coding agent from written plans and own the architecture, review and correctness myself. It is a personal product, not a public multi-user service, and it is honestly a better record of how I work than most things I could say about it.