We turn UK tax and compliance into plain-English toolkits you can run yourself. IR35, MTD, landlord, employer. Buy once. Saves days of admin.
Plug each toolkit into ChatGPT, Claude, or Gemini. One paste, audit-ready output, no generic guesses.
Each skill is a structured instruction set you drop into whichever chat tool you already use. Not a chatbot, not a wrapper. The 2025/26 HMRC bands, CEST criteria, and named UK case law are already inside the file. Three steps:
Pick the pack that matches your work. Each file covers one specific area of UK business compliance.
Tested on ChatGPT, Claude, and Gemini. Free tiers work. No API keys, no integrations, no setup.
The skill walks you through the inputs, runs the maths against current rates, and gives you the figure your accountant or HMRC actually wants.
Plain Markdown. Open it in any text editor. Drop it into your chat tool as context. Below: the first page of the IR35 Status Assessment skill.
Determine whether a contract falls inside or outside IR35 (off-payroll working rules) under HMRC's published guidance and the leading UK case law. Output: a written status decision, a CEST comparison, and a Status Determination Statement (SDS) ready to send.
ITEPA 2003 Part 2 Chapter 8, the IR35 statuteCEST tool decision logic, current at April 2026[2022] EWCA Civ 501[2024] UKSC 29[2022] EWCA Civ 502Walk the user through 14 questions covering: mutuality of obligation, right of substitution, control over work, financial risk, integration, exclusivity, intent and pay structure. Score against the three-stage Ready Mixed Concrete test as restated by the Court of Appeal in Atholl House.
Takes about 30 seconds. We'll highlight the pack we think suits you and show you the rest. You still decide.
Every skill is plain Markdown. It works in whatever chat tool you already use. This just helps us point you at the best starting format.
Buy the Suite if you wear more than one hat. Landlord plus freelancer. Employer plus contractor. Property pro running a side shop. If you're only one thing, the single pack below is the honest answer, save your £40. If you're three things at once, this pays for itself on the first deadline.
Not ready for the full suite? Start with the free IR35 checker, or grab one pack below to test the work.
Try the free IR35 checkerLifetime licence. Download once, use forever.
8 connected Notion modules. CRM, invoicing, tax dashboard, time tracking, HMRC deadlines. Your entire freelance business in one workspace.
ex-VAT. Gumroad adds 20% VAT for UK/EU buyers at checkout.
Tax calculator, IR35 assessment, contract review, invoice categoriser, proposal generator, client comms. Six files, one download. Covers Self Assessment 31 January 2026 and MTD for Income Tax (phase 1 live 6 April 2026 for >£50k turnover, phase 2 April 2027 >£30k, phase 3 April 2028 >£20k).
ex-VAT. Gumroad adds 20% VAT for UK/EU buyers at checkout.
Prefer us to run the IR35 assessment for you? £150 done-for-you review, 48hr turnaround →
Employment law, employee handbooks, health and safety risk assessments, GDPR compliance, AML/KYC documentation, contract review, client comms drafter. Seven files, one download. Updated for 2025/26 statutory rates (SSP £118.75, redundancy cap £719) and ERA 2025 commencement timing.
ex-VAT. Gumroad adds 20% VAT for UK/EU buyers at checkout.
Landlords of assured tenancies in England must comply with Renters' Rights Act 2025 transition rules by 31 May 2026. Financial penalty up to £7,000 per breach. The kit ships a Section 21 transition guide, a tenant information sheet template, a Ground 4A statement template, and the full grounds-for-possession quick reference. Plain English, ready to use.
ex-VAT. Gumroad adds 20% VAT for UK/EU buyers at checkout.
Landlord compliance (RRA 2025-aware), rental yield and SDLT calculator (post-1 April 2025 bands), GDPR compliance for tenant data. Built for England-based portfolio landlords.
ex-VAT. Gumroad adds 20% VAT for UK/EU buyers at checkout.
Every UK tax deadline, threshold, rate change, landlord reform milestone and IR35 tribunal citation a UK operator needs, in one live Sheet. Updated monthly with the latest HMRC, Companies House, ICO and landlord rule changes. 8 tabs, every figure sourced to gov.uk or BAILII with a last-verified date. Lifetime access, one payment.
ex-VAT. One-time payment, lifetime access, monthly updates. Gumroad adds 20% VAT for UK/EU buyers at checkout.
Because a generic chatbot is a starting point, not a finishing line. UK tax and compliance need more than a best guess.
11 discrete checks per IR35 run, versioned against 2025/26 HMRC bands. Methodology on GitHub.
We started ExternalOS in Leeds in early 2026 because we'd all done the same Sundays, the same HMRC tabs, the same 11pm panic before 30 January. Our team is small and built around the work itself: UK tax, employment law, contracting, landlord compliance, and the engineering to package it.
We spent months on the parts of running a UK business that nobody packages and everyone redoes. The HMRC bands. The CEST criteria. The Renters' Rights Act 2025. The Employment Rights Act 2025. We turned that work into files: one file per job, each one structured tight enough that the answer comes out the same every time, whoever opens it.
We charge £29 to £79 because the work is done. Not per user, not per month, not per filing. You download the file, you keep the file, and when the legislation moves we ship the update. That's the whole arrangement.
Plain-text, browser-based tools using the same case law, HMRC rates, and Companies House rules as the paid packs. Use them as much as you want. Buy if and when you want the rest.
Not ready to buy? Take the free IR35 skill file and see the work first. A new tool lands every Friday. Stop any time.
Send me the IR35 skillNo. A skill is a structured instruction set your chat tool loads before it answers you. It contains 2025/26 HMRC rates, CEST criteria, and named UK case law (Atholl House, PGMOL, Kickabout Productions) already embedded. A prompt is a sentence. A skill is an entire working system.
A plain-text instruction file (Markdown). About 4 to 12 pages of structured content per skill. Inside: the relevant HMRC rates and thresholds for 2025/26, the legislative references, the named case law, the step-by-step the tool walks you through, and the exact output format your accountant or HMRC software expects. You drop the whole file into your chat tool as context, then ask your question in plain English. The free IR35 checker shows you the same engine running in your browser.
Each skill is peer-reviewed against current UK case law and HMRC guidance before it ships. The IR35 logic cites Atholl House, PGMOL, and Kickabout Productions with paragraph references. Rates are tied to the 2025/26 HMRC bands. The source of the free IR35 checker is public on GitHub. If a figure ever moves, the update is shipped free under the lifetime licence. First-buyer feedback will be credited with names and roles in the Behind section once it lands. If you spot a mistake before we do, email hello@externalos.com and we will rebate your purchase price and patch the file the same day.
Yes, if you want someone to file your self-assessment, sign off your accounts, or represent you to HMRC. ExternalOS does the prep work an accountant would otherwise charge you for: the calculations, the categorisation, the IR35 check, the contract review. Most of our customers use both, and pay their accountant for less time.
Banks file what's already in your account. ExternalOS does the work that produces what gets filed: categorisation, IR35 status, expense classification, allowable deductions, and the maths that turns transactions into a quarterly summary. Banks own your account. We don't. The files are yours, on your computer, usable with any chat tool, with no platform dependency.
Gumroad adds 20% VAT at checkout for UK and EU buyers. That's the law (Gumroad is the registered merchant of record, they handle the VAT filing so we don't have to). Our listed prices (£29, £39, £79) are ex-VAT. If you're VAT-registered, you can reclaim it on your next return. If you're outside the UK and EU, no VAT is added.
If you can copy and paste, you can use it. Drop the skill file into whichever chat tool you already use, then ask your question in plain English. No API keys, no setup, no technical skills required.
You get the update. Lifetime licence means every rate change, legislation update, and CEST revision ships to you automatically. When the 2026/27 tax year drops, your skill already knows the new numbers.
Any modern chat tool that accepts a file or a long instruction set. No lock-in, no subscription, no platform dependency.
14-day refund, no questions asked. Email hello@externalos.com and it's sorted. You keep the files either way.
Drop your email, get the IR35 skill file to keep. Every Friday after that: a new free tool, a rate update, or a real use case. Stop any time.
First email lands within 60 seconds. IR35 skill file, plain Markdown, 8.4KB, yours to keep.
No spam. Unsubscribe in one click. We log the date and IP of your signup as proof of consent (UK GDPR).