Most subcontractors approach their tax return with a single objective: get the biggest refund possible. It is understandable. But for anyone who wants to buy a home, move to a bigger property, or remortgage in the next couple of years — aggressive expense claims can silently demolish your borrowing power before you even speak to a bank.

What Lenders Actually Look At

Here is the disconnect that catches most tradespeople off guard: you see your gross CIS income. Your bank sees your net taxable profit — the number left after every expense you have claimed has been deducted.

Mortgage underwriters do not care what hit your bank account. They care about your SA302 Tax Calculation — the document that HMRC produces after your Self Assessment is filed. That number is what they multiply to determine how much they will lend. The more you reduce it, the less house you can buy.

The Real-World Numbers

Two subcontractors. Same gross income. Very different financial futures.

Subcontractor B got a larger refund — perhaps £2,000 extra back from HMRC. In return, they lost £40,000 in mortgage capacity. That is not a tax strategy. That is a £38,000 mistake.

The Lender Red Flag You Cannot See Coming

It gets worse. Many lenders now scrutinise your expense-to-turnover ratio. If your claimed expenses look unusually high relative to what is typical for your trade, underwriters flag it — not just as a lending risk, but as a potential sign that the accounts are being manipulated to reduce tax.

The result is not just a lower loan offer. It can be an outright rejection, or a request for additional documentation that delays your purchase while another buyer moves ahead.

Legitimate, well-documented expenses are never the problem. The issue arises with borderline items — high-value vehicle leases with significant private use, equipment with dubious business justification, or costs that are hard to evidence. Claiming them reduces your tax and your creditworthiness simultaneously.

Checklist: How to Prepare Mortgage-Ready Accounts

• 01
  Plan 24 Months Ahead — Not 24 Days Most lenders require two, sometimes three, years of SA302s or finalised accounts. If you plan to buy or remortgage in 2027, your 2025/26 tax return matters just as much as 2026/27. Start managing your net profit figures now, not when you are already in mortgage conversations.
• 02
  Download Your SA302 Before You Submit HMRC's online portal lets you preview your tax calculation before you file. Look at your net profit figure and ask yourself: would a mortgage lender lend me what I need based on this number? If not, reconsider borderline expenses before the return is submitted — not after.
• 03
  Separate Essential from Optional Claims Prioritise genuine, well-documented business costs — tools, materials, fuel, PPE, training. Be cautious with high-value vehicle claims with mixed use, or expenses that are difficult to evidence. The savings on those borderline claims may cost you multiples in borrowing power.
• 04
  Use a Specialist Mortgage Broker High-street banks often struggle with CIS income and self-employment. A broker who specialises in construction workers and sole traders can access lenders with more flexible underwriting — some will consider gross income, average income over three years, or net profit plus add-backs. You may have more options than a standard bank quotation suggests.
• 05
  Talk to Your Accountant About the Trade-Off Explicitly Ask directly: "If I claim these borderline expenses, what does my SA302 look like — and what does that mean for a mortgage application?" A good accountant will model both scenarios. If they have never raised this issue with you unprompted, it is worth asking.

Decision Framework: Refund Now or Borrowing Power Later?

This is not a one-size-fits-all answer. The right choice depends entirely on where you are in life right now.

The Five-Minute Check Before You File

Before you finalise this year's Self Assessment, take five minutes to run this single calculation: multiply your expected net profit by four. Is that number enough to buy the home you want — or want to stay in?

If the answer is no, look at which expense claims are creating the gap. Are they genuinely essential, or are some borderline items that could sensibly be excluded — saving you the HMRC compliance risk and restoring your borrowing power at the same time?

A slightly smaller refund today can be worth tens of thousands of pounds in purchasing power tomorrow. It is worth five minutes of your time to find out which side of that line you are on.