Buy-to-Let Mortgages Explained: Everything Landlords Need to Know

Buy-to-let mortgage rules are fundamentally different from residential mortgage rules — and many first-time landlords discover this the hard way after making investment decisions based on incomplete information. In this guide, I cover how BTL mortgages are assessed, how much deposit you need, whether to buy through a limited company, what rental yield you need, and common mistakes that trip up both new and experienced landlords.

How is a buy-to-let mortgage different from a residential mortgage?

The differences are significant and affect almost every aspect of the product:

Deposit requirements: BTL lenders almost universally require a minimum 25% deposit, compared to 5% for residential mortgages. Some lenders accept 20% for standard properties with strong yields, but 25% is the baseline most buyers should plan for.

Affordability assessment: Where a residential mortgage is primarily assessed on your income, a BTL mortgage is primarily assessed on the expected rental income of the property. Lenders apply a rental stress test — typically requiring the rent to cover 125–145% of the mortgage interest payment, calculated at a notional stress rate (often 5.5% or higher, regardless of the actual rate). Your personal income may still be relevant, particularly for first-time landlords or where the property yield is borderline.

Interest rates: BTL mortgages typically carry higher interest rates than equivalent residential products, reflecting the greater risk the lender takes on investment property.

Interest-only availability: Interest-only BTL mortgages are far more widely available than interest-only residential products. Many landlords use interest-only to maximise monthly cash flow, relying on property value appreciation and eventual sale to repay the capital.

Tax treatment: BTL mortgage interest is no longer fully tax-deductible for individual landlords in the way it once was. Since the phased withdrawal of mortgage interest relief, individual landlords receive a basic rate tax credit rather than full deductibility. This has significantly changed the economics of BTL for higher and additional rate taxpayers — and is one reason many landlords now purchase through limited companies.

How much deposit do I need for a buy-to-let mortgage?

As a baseline, plan for 25% of the purchase price. On a £150,000 investment property, that is £37,500.

The deposit you put down affects both which lenders will consider your application and what rate you can access. At 75% LTV (25% deposit), you have access to a wide range of mainstream BTL lenders and competitive rates. At 80% LTV (20% deposit), the range of lenders narrows and rates are typically less competitive. Above 80%, BTL options become very limited.

For HMOs (houses in multiple occupation), multi-unit freehold blocks, or properties in non-standard construction, some lenders will require higher deposits — typically 30–35%. We always check lender criteria against the specific property before recommending a product.

How do lenders calculate how much I can borrow on a buy-to-let?

The key calculation is the Interest Coverage Ratio (ICR) stress test. Lenders take the expected monthly rental income and test whether it covers the mortgage interest at a notional stressed rate — typically 5.5% per annum or higher. The required coverage ratio is usually 125% for lower-rate taxpayers and 145% for higher or additional rate taxpayers (or for applications via limited companies, depending on the lender).

Here is a simplified example: if you want to borrow £120,000 interest-only at a stressed rate of 5.5%, the annual interest would be £6,600 — or £550 per month. At a 125% ICR, you would need £687.50 per month in rent to pass the stress test. At 145%, you would need £797.50.

This is why rental yield matters so much in BTL lending decisions — and why we always run these calculations before you make an offer on an investment property. A property that looks affordable on paper may not pass a lender's stress test, or may only work with a specific lender who uses a lower stress rate.

Do I need to already own a home to get a buy-to-let mortgage?

Most mainstream BTL lenders require you to be a homeowner — either owning outright or with a residential mortgage. They view homeownership as evidence of financial responsibility and property management experience. A smaller number of specialist lenders will consider first-time buyer BTL applications (sometimes called "first-time landlord" mortgages), but the rates and deposit requirements are typically less favourable, and the range of lenders is narrower.

If you are a first-time buyer who wants to invest in property rather than live in it, we can advise on what is available — but it is worth being realistic about the limitations upfront.

Can I get a buy-to-let mortgage through a limited company?

Yes, and this has become increasingly common following the reduction in mortgage interest tax relief for individual landlords. A limited company (specifically a Special Purpose Vehicle or SPV — a company set up specifically to hold property) can still deduct mortgage interest as a business expense, which makes the maths more attractive for higher and additional rate taxpayers.

The mortgage side of limited company BTL is straightforward — many lenders actively offer SPV mortgages. However, rates and arrangement fees are typically slightly higher than for personal ownership, and there are additional costs: company formation and annual filing, accountancy fees, and potential complexities when extracting profits from the company.

The decision between personal and limited company ownership is primarily a tax decision, and I always recommend clients speak to an accountant before making it. The mortgage implications are straightforward; the tax implications require professional advice tailored to your specific situation.

What rental yield do I need to qualify for a buy-to-let mortgage?

There is no single universal minimum yield — it depends on the lender's specific stress test, your tax status, and the LTV. However, as a rough guide:

• At 75% LTV with a 125% ICR at a 5.5% stress rate, you typically need a gross rental yield of around 5.5–6% to pass most lenders' tests
• At 75% LTV with a 145% ICR, you typically need closer to 6.5–7%
• Higher LTVs require higher yields to compensate for the larger loan size

Different lenders use different stress rates, which is one reason why the same property can pass one lender's criteria but fail another's. Knowing which lenders use more generous stress rates for certain property types or tax statuses is part of the broker value — we match your property's yield to the right lender rather than simply applying to the cheapest available rate.

Can I live in a property with a buy-to-let mortgage?

No. This is one of the most important points in BTL mortgage law. A buy-to-let mortgage is specifically for properties you do not intend to live in. If you live in a property with a BTL mortgage, you are in breach of your mortgage terms, and the lender can call in the debt.

The reverse also applies: a residential mortgage does not permit you to let out the property without the lender's consent. If you want to let your residential property, you need to either get the lender's permission (known as consent to let) or switch to a BTL mortgage. Letting a property without the lender's knowledge is mortgage fraud and carries serious consequences.

What happens to my BTL mortgage when my fixed rate ends?

Exactly the same as a residential mortgage — your lender will move you onto their Standard Variable Rate unless you act. BTL SVRs are typically even higher relative to market rates than residential SVRs, making it all the more important to remortgage before your fixed rate expires.

The BTL remortgage market is also an opportunity to release equity as your property value increases — which many landlords use to fund deposits for further purchases, to refurbish and revalue properties, or simply to extract capital from their portfolio.

Can I remortgage a buy-to-let property to release equity?

Yes, provided the rental income still passes the lender's stress test at the new, higher loan amount. This is a common strategy for portfolio landlords who want to grow their portfolios without selling existing properties — releasing equity from one property to use as a deposit on the next.

The calculations need to be run carefully — releasing equity increases your loan amount, which may push you into a higher ICR requirement at a higher stressed rate, potentially making it harder to pass the affordability test. We model these scenarios for landlord clients before any application is made.