What buy-to-let lenders look at

If you have only ever taken a residential mortgage, a buy-to-let application can feel like it speaks a different language. The lender spends less time on what you earn and far more on what the property is expected to earn. This is a plain account of what buy-to-let lenders weigh up, why it differs from a home loan, and where a readiness picture helps before you approach anyone.

The headline difference: the rent does the heavy lifting

For a residential mortgage, affordability hangs on your income and outgoings. For buy-to-let, the starting question is whether the expected rent comfortably covers the mortgage interest. Lenders express this as an interest coverage ratio (ICR): the rent measured against the monthly interest, at a stressed rate rather than the rate you would actually pay.

A common shape is an ICR of 125 percent for a basic-rate taxpayer and 145 percent for a higher-rate taxpayer, tested at a notional rate that is deliberately higher than today's pricing. The exact numbers vary by lender and change over time, so treat those figures as illustrative rather than a rule. The principle is steady: the rent has to clear the interest with a margin to spare, under conditions tougher than the present.

Why the stress rate matters

The stress rate is the lender asking a fair question: if interest rates were higher than they are now, would this still hold together? Testing the rent against a higher notional rate is how a lender builds in headroom. It is also why a property can price well at today's rate and still fall short on the assessment — the sum that matters is not the rate on the product, but the rate in the test.

This is the single most common surprise for first-time landlords. A property that looks affordable on a mortgage comparison site can come up short once the rent is measured against a stressed rate at the required coverage ratio.

Top-slicing: when personal income comes back in

Some lenders allow top-slicing, where surplus personal income is used to support a shortfall between the rent and the coverage requirement. It widens the range of properties that work, but it brings your own affordability back into the picture — so income, outgoings and existing commitments matter again. Not every lender offers it, and those that do apply their own conditions.

Portfolio landlords are assessed as a whole

Once you hold four or more mortgaged buy-to-let properties, most lenders treat you as a portfolio landlord and assess the whole portfolio, not just the property in front of them. Following the Prudential Regulation Authority's underwriting expectations, that typically means a view across total borrowing, aggregate rental cover and a business plan or asset schedule. The bar is higher and the paperwork is heavier, because the lender is underwriting a small business rather than a single purchase.

Regulated or not? A distinction worth knowing

Most buy-to-let lending is not regulated by the Financial Conduct Authority in the way a residential mortgage is, because it is treated as an investment rather than a home. The notable exception is consumer buy-to-let — for example, an accidental landlord letting a former home — which is regulated. The line is narrower than many people assume, and it changes the protections that apply, so it is worth confirming which side of it you sit on before you commit.

Where a readiness picture helps

None of the above is advice, and a readiness picture is not a decision. What it can do is let you see, before you approach a broker or lender, roughly where a prospective purchase stands: the expected rent against a stressed coverage requirement, your own income position if top-slicing might come into play, and the property and area context that underwriters will eventually look at.

Moxim builds that picture from sources you connect with read-only permission — Open Banking, income and property data — and presents it as an illustrative, educational summary you can take into a conversation. It is not a lending decision, and your broker still does the regulated work of recommending a product. The point is narrower and useful: fewer surprises at the stress-rate step, and a clearer sense of which properties are worth a closer look.

Moxim is a technology company, not a lender, and is not yet authorised by the Prudential Regulation Authority. Figures in this article are illustrative and for general education; they are not a quote, a recommendation, or a statement of what any lender will offer.