A landlord client earns well, has a large deposit and a clean credit file, and still gets offered a smaller loan than a first-time buyer on half their salary. Nothing has gone wrong. Buy-to-let affordability is calculated from a completely different starting point: the property has to pay for itself. The borrower's income sets whether they are eligible at all, but the size of the loan comes from the rent, tested against a rate nobody is currently paying, with a deliberate margin on top. That margin is the rental coverage ratio, and it is the number that quietly decides most investment purchases. This guide explains how the calculation works, what moves it, a worked example of the maximum loan, and how the assumed rent gets evidenced — the part that most often derails a case late.
A residential mortgage is repaid out of the borrower's earnings, so the lender models those earnings in detail — which is why residential affordability turns on income, commitments and an income multiple. A buy-to-let loan is repaid out of a tenancy. The landlord is the conduit, not the source.
So the lender asks a different question: does this property produce enough rent to service this debt with room to spare? Personal income still appears, usually as a minimum threshold for eligibility, sometimes as a rescue mechanism for a shortfall, and always as part of the underwriting picture. But a landlord earning three times more than another will not automatically be offered a larger loan on the same flat. The flat's rent is the constraint, and everything else negotiates around it.
The rental coverage ratio in plain terms
The interest coverage ratio — ICR, or simply rental cover — expresses rent as a multiple of the mortgage interest at a stressed rate. The test is short:
Monthly rent ÷ (monthly interest at the stress rate) ≥ required ratio
Required ratios commonly sit between 125% and 145%. The buffer is not arbitrary padding. Between the rent arriving and the mortgage being paid sit void periods, letting agent fees, maintenance, insurance, ground rent or service charges, and tax. A property that covers its mortgage exactly is a property that fails the first month a tenant moves out. The ratio is the lender's estimate of how much of the rent never reaches the loan.
Where the stress rate comes from
The second half of the formula matters as much as the ratio. Lenders do not test the payment at the rate on the product; they test it at a stressed rate designed to survive the end of the initial deal — often the product rate plus a margin, with a floor around 5.5% for shorter fixes. Fix for five years or more and many lenders stress at a materially lower rate, occasionally the pay rate itself, because the payment is contracted for longer.
That single rule is why the same landlord, same property and same rent can borrow noticeably more on a five-year fix than a two-year one. It is also why quoted maximum loans move whenever base rates move, in the same mechanical way described in how interest rates affect borrowing capacity.
A worked example: what €1,600 of rent actually borrows
Take an illustrative apartment expected to let for €1,600 per month. The landlord wants a €240,000 interest-only loan. The lender uses a stress rate of 7% and requires 145% coverage because the client is a higher-rate taxpayer.
Stressed annual interest: €240,000 × 7% = €16,800, or €1,400 per month.
Rent required at 145%: €1,400 × 1.45 = €2,030 per month.
Actual rent: €1,600. Coverage achieved: 1,600 ÷ 1,400 = 114%. The case fails.
Now run it backwards to find the loan the rent does support. Affordable monthly interest is €1,600 ÷ 1.45 = €1,103, so annual interest is €13,241, and at a 7% stress rate the maximum loan is 13,241 ÷ 0.07 ≈ €189,000.
The client asked for €240,000 and the property supports roughly €189,000 — a €51,000 gap that has to be filled with cash. On a €300,000 purchase that is the difference between a 20% deposit and a 37% one. Change one variable and the picture shifts again: at a 125% ratio (a basic-rate borrower with the same lender) the same rent supports about €219,000, and a five-year fix stressed at 5.5% instead of 7% would support roughly €240,000 at 145%. Same property, same tenant, three very different outcomes. (Figures are illustrative and rounded to show the mechanics; rates, ratios and criteria vary by lender and market.)
The levers that actually move the number
Once the arithmetic is clear, the advisory work is obvious: there are only four things to pull on.
The rent. The single biggest input, and the one the applicant has least control over once the valuer has spoken.
The product term. Longer fixes usually unlock a lower stress rate and therefore a bigger loan — at the cost of flexibility.
The coverage requirement. Driven by tax status and lender appetite, and one of the widest spreads in the market. Placement matters more here than pricing.
The loan size itself. Reducing the loan raises coverage and often drops the case into a lower loan-to-value band with a cheaper rate, which lowers the stressed payment again. The effect compounds pleasantly.
Top-slicing and the return of personal income
Some lenders offer top-slicing: where rental cover falls short, surplus personal income can absorb the gap. The lender runs a background affordability assessment — living costs, existing mortgages, commitments — and lends on the combination rather than the rent alone. It is a genuine solution for asset-rich, well-paid landlords buying in low-yield locations, where prime property routinely fails a straight ICR test.
Two cautions worth giving the client. First, top-slicing means the property is not self-financing, so a void period is paid for out of household income rather than absorbed by the rent buffer. Second, availability and criteria vary sharply, so it is a placement decision to make before an application, not a rescue to attempt after a decline.
Evidencing the rent — where cases die
Everything above runs on one assumption: the rent. And the landlord does not set it. The lender's valuer states a market rent based on comparable lettings, and the calculation uses that figure even if a tenant has already agreed to more. A valuer who comes back €100 a month under the expected rent knocks roughly €12,000 off the maximum loan in the example above — after the offer has been accepted, when there is least time to react.
The defence is to build the case before the valuation rather than argue after it: assemble comparable lettings and comparable sales for the street and the property type, and hand them over with the application. This is exactly the evidence pack Biedradar is designed to produce — enter an address and it returns comparable sales, a valuation range and local market signals, packaged as a branded property analysis report you can attach to a file or hand to a client in minutes.
What agents should take from this
For agents listing or selling investment stock, buy-to-let affordability is a qualification tool. An investor buyer's maximum is a function of the property's rent, so two facts decide whether their offer is real: the achievable rent and the deposit. Ask for both. An investor bidding at a price the rent cannot support is either top-slicing, paying largely in cash, or about to renegotiate after valuation.
It also changes how investment property should be marketed. A listing that evidences achievable rent with genuine comparables is easier to finance than one that quotes a hopeful yield, and easier-to-finance listings close more often. Pair that with a defensible sale price — the same rental valuation discipline applied to the purchase side — and the deal survives contact with the lender's valuer, which is where investment sales are usually lost.
Frequently asked questions
How is buy-to-let mortgage affordability calculated?
Mostly from the property's rent, not the landlord's salary. The lender takes the expected monthly rent, divides it by the monthly interest payment calculated at a stressed rate, and checks the result against a required rental coverage ratio — commonly in the 125% to 145% region depending on the borrower's tax position and the lender. If the rent does not cover the stressed interest by that margin, the loan is cut until it does. Personal income usually only sets a minimum eligibility threshold rather than the loan size.
What is the rental coverage ratio or ICR?
The interest coverage ratio is the expected rent expressed as a percentage of the mortgage interest at the lender's stress rate. A 145% requirement means the rent must be at least 1.45 times the stressed monthly interest. It exists because a landlord's ability to repay comes from the tenancy, and the lender wants a buffer for void periods, maintenance, letting fees and tax before the payment is at risk.
Why do lenders use a stress rate higher than the actual rate?
Because a buy-to-let loan will outlive the initial fixed period. The stress rate — often the product rate plus a margin, subject to a floor of roughly 5.5% on shorter fixes — models what the payment looks like when the deal ends and the loan reverts. Longer fixed terms of five years or more are frequently stressed at a lower rate, sometimes the pay rate itself, which is why a five-year fix can borrow noticeably more than a two-year fix on identical rent.
Does a higher-rate taxpayer get a smaller buy-to-let loan?
Often, yes, in markets where mortgage interest relief is restricted. Lenders apply a higher coverage requirement to borrowers in higher tax bands because more of the rent is lost to tax before it reaches the mortgage payment. The same property and the same rent can therefore support a materially different loan for two landlords with identical deposits, purely because of their marginal tax rate.
What is top-slicing on a buy-to-let mortgage?
Top-slicing lets a landlord use surplus personal income to make up a shortfall in rental coverage. Instead of cutting the loan to the level the rent supports, the lender tests whether the borrower's other income comfortably absorbs the gap after their own living costs and commitments. It is not universally offered, it requires evidenced income, and it usually comes with tighter background affordability checks — but it can rescue a case where the rent falls just short.
Who decides the rent a lender uses?
The lender's valuer, not the landlord and not the selling agent. The valuation report states a market rent based on comparable lettings, and the affordability calculation runs on that figure even if the applicant has a tenant lined up at more. This is why a case can pass on paper and fail on the valuation: an optimistic rent assumption of a hundred a month can move the maximum loan by tens of thousands.
