How to file a CT600 for your buy-to-let company: a plain-English walkthrough

Updated July 2026

"Filing a CT600" is shorthand for filing a full Company Tax Return, which is actually three documents submitted together to HMRC:

Document	What it is
The CT600 form	The return itself — a set of numbered boxes summarising your profit and tax.
The tax computation	How you got from the profit in your accounts to the taxable profit: adjustments like adding back depreciation and deducting mortgage interest.
The statutory accounts	For most small buy-to-let companies, FRS 105 micro-entity accounts: a profit and loss account and a balance sheet.

The accounts and computation must be tagged in iXBRL — a machine-readable format HMRC's systems check on receipt. This is why you can't just post a PDF, and why (since HMRC's free service closed in March 2026) you need software to file.

What you need before you start

For a simple buy-to-let company, everything fits on one sheet of paper: your company registration number (on Companies House), your 10-digit Corporation Tax UTR (on letters from HMRC), your accounting period dates, and the year's totals — rent received, letting agent fees, repairs, insurance, other expenses, and mortgage interest (interest only, not the capital part of your payments). For the balance sheet you'll need the property's cost, the mortgage balance at the period end, the bank balance, and anything owed to or by the company at that date — including money you've lent the company yourself.

If you kept your records in a spreadsheet all year this takes minutes. If not, our free SPV bookkeeping spreadsheet is built so its category totals map one-to-one onto the questions our filing wizard asks.

The deadlines that matter

Two different deadlines run from your accounting period end, and they are in an odd order: the payment is due first, 9 months and 1 day after the period end; the return is due 12 months after. So for a period ending 31 March 2026, tax is payable by 1 January 2027 and the return must be filed by 31 March 2027. Filing early is the sensible way to know what to pay.

Don't confuse this with Companies House: your accounts must also be filed there (within 9 months of the period end), and that is a separate submission this tool doesn't make — though we show you exactly how after you file with HMRC.

Filing with this site, step by step

The wizard starts with a 60-second eligibility check — three quick checks with no signup. It exists because we only handle the simple case: a UK company whose only activity is letting residential property it owns, no property bought or sold, no group, no money borrowed from the company by directors. Loss years are handled (the loss carries forward automatically), and so is a long first period — 12 months plus a few days — where we prepare both required returns from the one interview. If anything doesn't fit, we tell you why and what to do instead.

Then six short sections: company details, period, rent and expenses, mortgage and loans, balance sheet, declaration. As you type, the right-hand panel shows the live result — property profit, mortgage interest deduction, taxable profit, marginal relief if any, and the corporation tax to pay. Every guardrail is checked as you go; if something puts you outside scope, you're told immediately rather than at the end.

Finally you download your filing pack — the real iXBRL accounts, the tax computation, and the CT600 values — and submit to HMRC. Keep the pack with your company records.

When not to do it yourself

Some years genuinely need an accountant, and it is cheaper to accept that early: the year you buy or sell a property (chargeable gains), an overdrawn director's loan account (s455 tax), or anything involving groups or non-residential lets. Our eligibility gate screens for all of these — that's the point of it.