Top CGT Mistakes HMRC Sees Most Often
Top CGT Mistakes HMRC Sees Most Often
HMRC doesn't challenge every CGT return — just those where the numbers or documentation don't stack up. Many disputes are avoidable with better planning and record keeping.
This guide highlights the most common errors HMRC identifies in Capital Gains Tax returns for UK property, explained in plain English with clear examples of what HMRC is actually looking for.
Mistake 1: Treating Routine Maintenance as a Capital Cost
Why it matters
Only capital improvements — works that enhance the value of the property permanently — are allowable in the CGT cost base. Routine maintenance simply keeps the property in its existing condition.
HMRC looks for
- Receipts with clear descriptions
- Distinction between repairs and enhancements
Example
Replacing carpet in a single room usually isn't a capital cost. Installing high-spec flooring throughout as part of an upgrade might be, if you can show the improvement persists at sale.
Mistake 2: Using Retrospective or Unsupported Valuations
Why it matters
Valuation is critical when property isn't sold on an open market (e.g., gifts, transfers, inheritance). HMRC expects methods that align with market realities at the relevant date.
HMRC looks for
- Professional valuations where value is contested
- Market comparables from the relevant period
Example
A valuation "based on my opinion" carried out years after acquisition or disposal is usually unreliable in HMRC's view.
Mistake 3: Claiming All Costs, Including Those Already Used Elsewhere
Why it matters
Some costs may reduce rental income tax but cannot be used again to adjust the CGT base cost. HMRC checks for double claiming.
HMRC looks for
- Cross-check between rental income deductions and CGT cost bases
Example
Agent fees claimed against rental income but also included as 'selling costs' need clear separation.
Mistake 4: Misunderstanding Reliefs (Especially Residence and Letting Relief)
Why it matters
Reliefs can significantly reduce gains, but HMRC applies strict conditions.
HMRC looks for
- Evidence of actual occupation
- Accurate apportionment of time
Example
Claiming Lettings Relief without having qualified for Principal Private Residence Relief first is a common rejection.
Mistake 5: Assuming "No CGT Change in Budget" Means "No CGT Risk"
Why it matters
Even when a Budget doesn't alter CGT rates, HMRC still enforces existing laws. Many investors assume that if the rate does not change, CGT is not worth thinking about — that assumption is costly.
HMRC looks for
- Correct application of unchanged rates
- Accurate recording of gains even if tax is unchanged
Example
Entering zero for CGT because no change was expected from a Budget announcement, without actually calculating a gain.
Mistake 6: Inadequate Documentation at Disposal Time
Why it matters
HMRC bases assessments on the documentation supplied. Inadequate or missing evidence is one of the top triggers for CGT enquiries.
HMRC looks for
- Clear cost breakdowns
- Dated purchase and sale documents
- Invoices for capital improvements
Example
Responding to an enquiry with bank screenshots rather than formal invoices often leads to disallowance.
Mistake 7: Ignoring Reporting Deadlines
Why it matters
For UK residential property, the CGT reporting and payment deadline is time-bound (generally 60 days post-sale). Even zero-tax outcomes must be reported.
HMRC looks for
- Timely submissions
- Evidence of submission date
Example
Late reporting because "the gain looked small" still triggers penalties.
Mistake 8: Not Understanding Company vs Individual Treatment
Why it matters
Individuals and companies use different tax regimes. Assuming "no CGT for companies" without understanding the corporation tax consequences is risky.
HMRC looks for
- Correct identification of ownership structure
- Appropriate computation under the correct regime
Example
Treating a company-held property disposal as if individual CGT applied leads to incorrect filings and unnecessary risk.
Final Takeaway: Documentation, Classification, and Consistency
HMRC disputes are rarely about disagreement over law — they are about documentation, classification, and consistency. Investors who plan, record, and categorise at the time of the transaction reduce their risk and simplify compliance.
Frequently Asked Questions
How do I know if a cost is capital or revenue for CGT purposes?
Capital costs create or enhance the asset permanently (extensions, loft conversions, material upgrades). Revenue costs maintain existing condition (routine repairs, replacing worn items with similar quality). Categorise costs immediately when they occur with clear receipts and descriptions. When in doubt, consult our CGT guide.
What makes a valuation acceptable to HMRC?
HMRC expects professional valuations or clear market comparables from the relevant period. Retrospective estimates based on memory or opinion years after the fact are usually rejected. Record contemporaneous evidence for all non-arm's-length transactions (gifts, transfers, inheritance).
Can I claim the same cost against both rental income tax and CGT?
No. Some costs reduce rental income tax but cannot be used again to adjust CGT base cost. HMRC cross-checks between rental income deductions and CGT cost bases. Maintain separate line items for revenue expenses (rental tax) vs capital CGT costs. Use the Rental Yield Calculator to track operating expenses separately.
What evidence do I need to claim Private Residence Relief or Lettings Relief?
HMRC requires evidence of actual occupation and accurate time apportionment. Document occupancy dates and rationale at the point of entry. Note: Lettings Relief requires you to have qualified for Principal Private Residence Relief first. Claiming Lettings Relief without PPR qualification is commonly rejected.
If the Budget didn't change CGT rates, do I still need to calculate gains?
Yes. HMRC still enforces existing laws even when rates don't change. Always perform the full gain calculation—entering zero because "no change was expected" is a common and costly mistake. Visit our Budget 2025 Hub to understand what actually changed.
What documentation should I keep for CGT purposes?
Keep clear cost breakdowns, dated purchase and sale documents, formal invoices for capital improvements, and professional valuations. Bank screenshots are insufficient—HMRC expects formal invoices. Archive key documents systematically at the time of transaction, not retroactively.
What is the CGT reporting deadline for residential property?
For UK residential property, CGT reporting and payment must be made within 60 days of completion (not exchange). This applies even if no tax is due. Late reporting triggers penalties regardless of gain size. Set calendar reminders at sale completion.
Do companies pay CGT on property disposals?
No. Companies pay Corporation Tax on gains, not CGT. The regimes differ significantly in calculation mechanics and planning considerations. Treating a company-held disposal as individual CGT leads to incorrect filings. Review our Limited Companies guide for structure comparison.
Stay Updated
Subscribe to our weekly briefings for curated property news and insights