Unsupported Tax Features

TinyTax is designed for straightforward filings. Learn what advanced CT600 features aren't supported and how to work around limitations using existing fields.

TinyTax is designed for straightforward company filings. Some advanced CT600 features are not yet supported as dedicated fields. This guide explains what's not available and how to work around common situations.

Select your company type to customise this guide:

Trading Company

Property Company

Mixed Income

What TinyTax Supports

TinyTax handles the most common filing scenarios:

Trading income (Box 145, 155, 165)
Annual Investment Allowance (Box 735) - up to £1,000,000 per year
Trading losses - brought forward and carried forward
Interest income (Box 170)
Disallowable expenses - entertaining, fines, depreciation add-back
Marginal relief calculations

Property rental income (Box 190)
Annual Investment Allowance (Box 735) - for equipment and fixtures
Property losses - brought forward and carried forward (commercial-basis treatment only — see note below)
Interest income (Box 170)
Disallowable expenses - depreciation add-back
Marginal relief calculations

ℹ Property losses are relieved against your total profits (including interest income), on the basis that your property business is run on a commercial basis (CTA 2010 s.64). TinyTax cannot ring-fence losses for a non-commercial property business — for example a members' club or association letting at a nominal rent — where the loss may only be carried forward against future rental income. See "UK Property Business Losses on the CT600" for details.

Workarounds Using Existing Fields

For many situations, you can achieve the correct tax payable using TinyTax's existing fields. The key is the Other adjustments field and Annual Investment Allowance.

How to Access These Fields

The Tax computation section groups its more specialist rows behind chevrons (collapsed by default). The most relevant for the workarounds in this guide is the Specialist adjustments chevron — click Show ▾ next to that heading to reveal:

Field	Box	Use For
Other adjustments (add if positive, deduct if negative)	—	Any adjustments not covered by other fields
Less: Annual Investment Allowance	735	100% first-year allowances on qualifying equipment
Add back: Depreciation	—	Automatically populated from P&L
Add back: Disallowable expenses	—	Client entertaining, fines, penalties

ℹ Tip — clicking Show all at the top of the Tax computation section opens every chevron at once if you want to see all the specialist fields together.

When to Use Specialist Software

TinyTax may not be suitable if your company has:

Complex capital allowances - multiple pools, balancing charges, or short-life assets
R&D activities (the CT600L supplementary page) - claiming R&D tax credits, whether SME R&D relief or RDEC. CT600L is HMRC's Research & Development supplementary page; TinyTax does not produce it. (Note: CT600L is for R&D — not loan relationships. Non-trade loan relationship deficits go on the main CT600 — Box 260 for a deficit arising this period, Box 230 for one brought forward — both of which TinyTax does support.)
Group structures - group relief claims between companies
Northern Ireland trading - separate NI profits calculation
Investors' relief or BADR - Box 215 or Box 220 gains
Non-commercial property businesses - where the letting isn't on a commercial basis (e.g. a club letting at nominal rent), property losses must be ring-fenced to future rental income rather than set against other income; TinyTax always relieves them against total profits

In these cases, consider:

Full CT600 software (TaxCalc, Taxfiler, etc.)
Using an accountant who can file on your behalf
HMRC's own CT600 filing service (basic but comprehensive)

Last updated: June 2026

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