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Buy-to-let affordability explained (UK): ICR, stress rates and typical lender assumptions

Learn how UK buy-to-let affordability is often assessed using ICR (rental cover) and a stress rate. Includes worked examples and common pitfalls.

Published: 13/03/2026 • Last verified: 13/03/2026

Summary

When you apply for a buy‑to‑let mortgage in the UK, lenders often run an affordability check based on:

  • a stress rate (a higher test interest rate), and
  • a rental cover / ICR requirement (rent must exceed stressed interest by a margin).

That’s why a property’s rent can “cap” how much you can borrow, even if you have a big deposit.

Use the buy‑to‑let calculator as you read:

This guide is general information only. Lender criteria vary and can change at any time.

Key terms (quick definitions)

How it works

Although every lender’s model is different, the logic often looks like this:

  1. The lender chooses a test interest rate (the stress rate).
  2. The lender calculates the stressed interest on the proposed mortgage amount.
  3. The lender applies an ICR requirement (for example, rent must be 125%–145% of stressed interest, depending on policy).
  4. Your expected rent must meet or exceed that required rent.

Two practical outcomes:

  • If the expected rent is fixed (because the market rent is what it is), it can cap the mortgage amount.
  • If the mortgage amount is fixed (because you want a particular loan), it can require a minimum rent.

Why requirements vary

You’ll often see different affordability outcomes for the same property because lenders can differ on:

  • how they account for borrower tax position and costs,
  • what stress rate they use,
  • what ICR % they require,
  • whether they use interest‑only for the test (common) or model capital repayment too.

The Bank of England has discussed how buy‑to‑let lenders can account for tax differently in affordability, which is one reason outcomes can vary between lenders.

Worked examples

These examples are illustrative to show the mechanics. Use the calculator for your own inputs:

Example 1: “How much rent is required?”

Assume:

  • Mortgage: £200,000
  • Stress rate: 6% (illustrative)
  • Interest‑only test (common for ICR models)
  • ICR requirement: 130%

Step 1: Stressed annual interest:

  • (£200,000 × 0.06 = £12,000) per year

Step 2: Stressed monthly interest:

  • (£12,000 / 12 = £1,000) per month

Step 3: Required rent at 130%:

  • (£1,000 × 1.30 = £1,300) per month

If market rent is £1,150/month, that shortfall can limit the borrowing.

Example 2: “My rent is £1,100/month — what mortgage could that support?”

Assume:

  • Rent: £1,100/month
  • ICR requirement: 130%
  • Stress rate: 6%

Step 1: Maximum stressed interest allowed:

  • (£1,100 / 1.30 ≈ £846) per month
  • Annual equivalent: (£846 × 12 ≈ £10,152)

Step 2: Convert to a loan size at 6%:

  • (£10,152 / 0.06 ≈ £169,200)

So even if you want a £200,000 mortgage, the rent might cap you closer to ~£169,000 under these assumptions.

Example 3: “Same property, different ICR requirement”

Keep Example 1’s stress rate and loan, but compare ICR:

  • Stressed monthly interest: £1,000
  • At 125% required rent: (£1,000 × 1.25 = £1,250)/month
  • At 145% required rent: (£1,000 × 1.45 = £1,450)/month

That single policy difference can shift the maximum borrowing — even before you consider stress‑rate differences.

Common mistakes

  • Assuming one “standard” ICR: treat ICR as lender‑specific and variable.
  • Mixing up yield with affordability: a good yield does not guarantee the rent will pass the lender’s stress test.
  • Forgetting voids and costs: lenders may not model your exact costs; you still need to stress‑test your own cash flow.
  • Using today’s pay rate instead of the stress rate: affordability checks often use a higher test rate.
  • Ignoring property‑specific constraints: some property types can face tighter criteria at high LTV.
  • Confusing “passes affordability” with “good investment”: passing a stress test doesn’t mean the numbers work for your goals.

What to do next

Related glossary:

Sources