Many buy-to-let deals are worked out on a simple assumption: put down 25%, borrow the other 75%. But a lender will only lend 75% if the rent is high enough to pass its stress test. If it isn't, the loan is cut, and you need a bigger deposit than you planned — sometimes tens of thousands of pounds more.
This guide explains how buy-to-let stress tests work and how to check a deal before you pay a sourcing fee or a solicitor.
This article is general information, not financial or mortgage advice. Lender criteria vary and change often. Speak to a whole-of-market mortgage broker before relying on any figure.
The two limits on a buy-to-let loan
A buy-to-let lender caps the loan at the lower of two figures:
- Loan to value (LTV): a percentage of the property's value, commonly up to 75%.
- The rental calculation: the largest loan the rent can support once the lender applies an interest cover ratio and a stress rate.
Deal packs usually show the first limit. The second is the one that catches investors out.
Interest cover ratio (ICR)
The ICR is how much the rent must exceed the mortgage interest. An ICR of 125% means the annual rent must be at least 1.25 times the annual interest, calculated at the lender's stress rate.
Typical starting points are:
- Around 125% for limited companies and for individuals who pay basic-rate tax.
- Around 145% for individuals who pay higher or additional-rate tax. Since Section 24 restricted mortgage interest relief, lenders expect higher-rate landlords to need more headroom.
Lenders set their own figures, and some use different ratios for HMOs, holiday lets or particular products.
The stress rate
Lenders don't test the rent against the rate you'll actually pay. Under Bank of England (PRA) rules for buy-to-let lending, they must check the loan stays affordable if rates rise. In practice:
- On shorter fixes and trackers, most lenders test at the higher of a minimum rate (often around 5.5%) or your pay rate plus about 2 percentage points.
- On fixes of five years or more, many lenders test at the actual pay rate. That's why five-year fixes often let you borrow more on the same rent.
The formula
Maximum loan = annual rent ÷ (ICR × stress rate)
With rent of £950 a month, an ICR of 125% and a stress rate of 5.5%:
£11,400 ÷ (1.25 × 0.055) = £11,400 ÷ 0.06875 = £165,818
Worked example: when the rent doesn't stack
A deal pack offers a two-bed terrace at £180,000. A 75% mortgage would be £135,000, with a £45,000 deposit.
If the rent is £950 a month:
- At 125% ICR the rent supports up to £165,818. The £135,000 loan passes.
- At 145% ICR (a higher-rate taxpayer buying personally) it supports £11,400 ÷ (1.45 × 0.055) = £142,947. Still passes.
If the lender's valuer says the rent is really £750 a month:
- At 125% ICR: £9,000 ÷ 0.06875 = £130,909. The loan is cut by about £4,100, so the deposit rises to around £49,100.
- At 145% ICR: £9,000 ÷ 0.07975 = £112,853. The loan is cut by about £22,100, and the deposit becomes roughly £67,100 — nearly half as much again as planned.
Nothing about the property changed. Only the rent figure and the borrower's tax position did.
Whose rent figure counts
The rent in the deal pack is not the rent the lender uses. The lender's valuer gives their own opinion of market rent, usually for a standard single let on an assured shorthold tenancy. That figure can come in below the deal pack's, especially when:
- the deal pack quotes the best rent in the area rather than typical achieved rents;
- the rent assumes a refurbishment that hasn't happened yet;
- the figure is room-by-room HMO income, but the lender values the property as a single let;
- the income is from serviced accommodation, which most standard buy-to-let lenders won't use.
To sanity-check rent yourself, look at let-agreed listings for the same property type and street on the portals, and ask a local letting agent what similar homes actually let for. Our guide to calculating yields shows how a lower rent flows through to returns.
Ways investors bridge a shortfall
- A bigger deposit. The simplest fix, if you have the cash.
- A five-year fix. Testing at the pay rate rather than a stressed rate can raise the maximum loan noticeably.
- Buying in a limited company. Often tested at 125% rather than 145%, but with other costs and trade-offs. See our Section 24 guide.
- Top slicing. Some lenders let individuals use spare personal income to cover a rent shortfall. Criteria are strict and not every lender offers it.
- Specialist HMO lenders. These may assess HMO income room by room, if the property is licensed and set up as an HMO.
BRR deals: the refinance is a stress test too
A BRR plan only works if the refinance releases the cash you expect. That new mortgage faces the same rental calculation, based on the post-refurb valuation and the post-refurb rent. A higher end value doesn't help if the rent can't support the larger loan. Many lenders also want you to have owned the property for six months before lending on a new valuation. Factor both into your timeline. More in BRR deals: the checks to make.
Where stress tests don't apply
Rent-to-rent and rent-to-serviced-accommodation deals don't involve a buy-to-let mortgage, so these tests don't apply. Portfolio landlords with four or more mortgaged buy-to-let properties face extra checks: lenders look at the whole portfolio's cash flow, not just the new property.
Checklist before you commit to a BTL or BRR deal
- What rent does the deal assume, and is it achieved rent for a single let?
- Will you buy personally or through a company, and what is your tax band?
- Run the formula at 125% and 145% ICR with a 5.5% stress rate. Does the loan you need still pass?
- If the rent came in 10–20% lower, how much more deposit would you need, and do you have it?
- For BRR, have you run the test on the post-refurb rent, and allowed for the six-month wait?
- Have you spoken to a broker before paying a sourcing fee?
When you browse buy-to-let deals or BRR deals, run this check alongside verifying the market value and budgeting for stamp duty. Together they tell you whether a deal works with the money you actually have.