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Tax Planning

When to File Your Self Assessment for Optimal Mortgage Timing

Understanding the 18-month document validity rule and how to time your tax return filing to maximize your mortgage application window.

Updated January 20268 min read

The 18-Month Validity Rule

One of the most confusing aspects of self-employed mortgages is document validity. Lenders require your income evidence to be "current" — but what does that mean?

The key rule

Your SA302 and tax calculations are typically valid for 18 months from the end of the tax year they relate to — not from when you filed them. The UK tax year ends on 5 April.

This means a 2024/25 tax return (covering the year ending 5 April 2025) is valid until approximately October 2026, whether you filed it in April 2025 or January 2026.

Why This Matters

  • Earlier filing = longer usable window — File as soon as possible after 6 April to maximize the time you can use your documents
  • Mortgage completions take time — Allow 3-4 months from application to completion; your documents must remain valid throughout
  • Income dips affect strategy — If your latest year was weak, you might want to apply before that year's figures become required

Tax Year Timing Calendar

Use this table to understand when your tax documents will be valid for mortgage applications:

SA302 validity periods for recent tax years
Tax Year
Tax Year Ends
Earliest Filing
Filing Deadline
Documents Valid Until
Optimal Application Window
2024/255 April 20256 April 202531 January 20265 October 2026April 2025 - October 2026
2025/265 April 20266 April 202631 January 20275 October 2027April 2026 - October 2027
2026/275 April 20276 April 202731 January 20285 October 2028April 2027 - October 2028

Document validity varies

While 18 months is the common rule, some lenders apply stricter windows (12 months) or more flexible ones. Always confirm with your broker or lender.

Strategic Filing Decisions

When to File Early (April-July)

File early if:

  • • Your latest year shows strong or improved income
  • • You want maximum flexibility on when to apply
  • • You're ready to start the mortgage process soon
  • • You want time to correct any errors before applying

When to File Later (Autumn-January)

Consider delaying if:

  • • Your latest year shows lower income than previous years
  • • You want to apply using the previous year's figures (before the new return is required)
  • • You're not ready to apply until well into the following year anyway

The Income Dip Strategy

If your 2024/25 income was significantly lower than 2023/24, you have a strategic choice:

Option A: Apply Before Filing

Apply for your mortgage before filing your 2024/25 return. Lenders will use your 2023/24 figures (and possibly 2022/23).

Best if: Your older years show stronger income and you're ready to buy soon.

Option B: Wait and Include New Year

File your return and apply using the new figures. Some lenders average years, which might smooth out the dip.

Best if: You're using a lender that averages multiple years, or your income is recovering.

Common Scenarios

1

Steady income across years

Situation: Your income has been consistent (£60k, £62k, £58k over 3 years).

Strategy: File as early as possible to maximize your application window. Lender choice won't dramatically affect your borrowing since figures are similar.

2

Recent income growth

Situation: Your income has grown (£45k, £55k, £70k over 3 years).

Strategy: File your latest year early and seek lenders who use the latest year only or weight recent years more heavily. This maximizes your borrowing.

3

Recent income drop

Situation: Your income dropped (£70k, £65k, £50k over 3 years).

Strategy: Consider applying before the weak year becomes your "latest year" for lenders. Alternatively, find lenders who average multiple years to smooth the dip.

4

One-off exceptional year

Situation: One year was unusually high or low (£55k, £120k, £60k over 3 years).

Strategy: If the spike is in your latest year, use lenders who take the latest year. If it's the dip, use lenders who average. Document reasons for anomalies.

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