A buy-to-let mortgage is not a bigger version of the mortgage you took on your home. It is a piece of business finance, priced, sized and underwritten on the income the property produces rather than on your salary. Understanding that one shift explains almost everything else: why the loan is usually interest-only, why the rent (not your job) decides how much you can borrow, why a limited company borrows differently from an individual, and why a case declined by one lender is often approved by the next.
This guide is the finance-mechanics hub for landlords deciding how to fund a purchase or refinance. It covers how lenders size a loan through the interest coverage ratio and stress test, how much deposit you need, how personal-name and company/SPV borrowing differ, the interest-only question, what drives the rate, and where the special cases sit. It is deliberately not the tax page. The ownership and Section 24 decision is summarised here in a few sentences and cross-linked to the pages that own it in full, so you can read the finance and the tax side by side without either one being thin.
What a buy-to-let mortgage is, and how it differs from a residential mortgage
A residential mortgage funds a home you occupy. It is a regulated consumer contract under the Financial Services and Markets Act 2000 (Regulated Activities) Order 2001, and the lender assesses it mainly against your personal income, spending and credit profile. A buy-to-let mortgage funds a property you let to tenants for a return. Most buy-to-let lending to companies and professional investors is unregulated business lending, assessed on the rental economics of the asset.
The practical differences follow from that:
- Sized from rent, not salary. The headline test is whether the rent covers the interest with a margin, using the interest coverage ratio below. Your personal income is often a secondary floor, not the main gate.
- Usually interest-only. Most landlords take interest-only so the monthly cost stays low and the rental cover is easier to pass, then repay the capital by selling or remortgaging later.
- Higher rates and bigger deposits. Buy-to-let rates sit above residential rates, and the deposit is normally 25% rather than the 5% to 10% common on a home.
- Different tax treatment. The interest is a finance cost restricted by Section 24 for individuals but fully deductible for companies, which is the single biggest reason the company/SPV route exists.
The dividing line that matters legally is occupation. If you, or a person related to you, occupy or will occupy the property, the loan is a regulated mortgage contract under Article 61 of the Regulated Activities Order, not a standard buy-to-let, and it sits outside the scope of this guide. See the FCA's perimeter guidance in PERG 4.10A on buy-to-let mortgages and the definition in the Regulated Activities Order 2001. Where a scenario crosses that line (a let-to-buy onward home, an expat buying a family home), we flag it and point you to an FCA-authorised mortgage adviser rather than treat it as something we cover.
How lenders decide how much you can borrow: ICR and the stress test
The core of buy-to-let underwriting is the interest coverage ratio (ICR). The lender wants the rent to exceed the mortgage interest by a set margin so the loan still stands if rates rise or a void hits. Two numbers drive it, both grounded in the Prudential Regulation Authority's SS13/16 underwriting standards for buy-to-let:
- The ICR percentage. Typically 125% for a limited-company/SPV borrower and for a basic-rate individual, and 145% for a higher or additional-rate individual. The higher figure for higher-rate individuals reflects the Section 24 finance-cost restriction, which raises their real tax cost on the interest.
- The stress rate. Lenders do not test against the pay rate. They test against a higher assumed rate, commonly around 5.5%, or the product rate plus 2%, whichever is higher, for two-year and variable products. Five-year-plus fixes and pound-for-pound remortgages (no extra borrowing) are often stressed lower, sometimes around 5% or the pay rate. Treat these as typical, lender-specific and worth verifying at the time.
Put together, the maximum loan is: annual rent, divided by the ICR, divided by the stress rate. On an £1,000 monthly rent (£12,000 a year) at 125% and 5.5%, the maximum interest the lender will allow is £12,000 / 1.25 = £9,600 a year, so the maximum loan is £9,600 / 0.055 = roughly £174,500. Flip the ICR to 145% for a higher-rate individual and the ceiling drops to about £150,500 on the same rent. Same property, same rent, but the borrower type moves the borrowing power by around £24,000. You can model your own case with the buy-to-let rental stress test calculator.
A worked example: why the ICR gap is the reason SPV lending exists
Take a £200,000 flat renting at £1,000 a month, bought at 75% loan-to-value, so a £150,000 mortgage and a £50,000 deposit.
At the 5.5% stress rate, the £150,000 loan carries assumed interest of £8,250 a year, which is £687.50 a month. Now apply the two ICR bands:
| Borrower | ICR | Monthly rent the £150k loan needs | Result on £1,000 rent |
|---|---|---|---|
| Limited company / SPV, or basic-rate individual | 125% | £687.50 × 1.25 = £859 | Clears, with £141 of monthly headroom |
| Higher-rate individual | 145% | £687.50 × 1.45 = £997 | Scrapes through, with only £3 of headroom |
The £150,000 is comfortably affordable inside a company at 125%. For a higher-rate individual at 145% it is right at the ceiling: a £50-a-month lower rent, or a small rise in the stress rate, and the same borrower is declined at £150,000 while the company still lends. That gap is not a quirk. It is the ICR working as designed, and it is the practical reason a large share of new buy-to-let demand routes through a special purpose vehicle. The company can either borrow the same amount with more cover, or borrow more against the same rent.
Note the pattern well: the finance did not change. The property, the rent and the deposit are identical. Only the ownership wrapper moved the outcome, and that wrapper is a tax and structuring decision before it is a lending one.
Deposit and loan-to-value
Loan-to-value (LTV) is the loan as a percentage of the property value, and it sets the deposit. The buy-to-let norm is 75% LTV, so a 25% deposit. Products up to 80% LTV (a 20% deposit) exist at higher rates and tighter rental cover, and specialist or higher-risk cases (some holiday lets, some expat borrowers, larger houses in multiple occupation) commonly want 30% or more.
A larger deposit does more than clear the LTV cap. Because it shrinks the loan, it also cuts the assumed interest in the stress test, which makes the ICR easier to pass and can move you into a lower rate band. On a marginal rental case, finding another 5% of deposit is sometimes the difference between a decline and an approval, and between two rate tiers. Deposit and LTV mechanics, including where the money can come from, are covered in depth in the buy-to-let deposit and mortgage requirements guide.
Personal name versus limited company or SPV finance
This is where finance and tax meet, and it is worth being precise about which side is which. On the finance side, an individual and a company borrow in broadly the same way: the same 75% LTV bands, the same stress test, the same interest-only norm. The company is often a small rate premium above personal borrowing, and it borrows at the friendlier 125% ICR rather than 145%.
On the tax side, the difference is large. Under Section 24, an individual landlord no longer deducts mortgage interest from rental profit; they receive a 20% basic-rate tax credit instead. A limited company deducts interest in full against profit before corporation tax, because Section 24 is an income-tax rule that does not touch companies. See HMRC's Property Income Manual at PIM2050 for the finance-cost treatment. That single asymmetry is why higher and additional-rate landlords model an SPV, and it is what the ICR bands above quietly reflect.
It is not a free win. A company carries running costs, extracting profit is taxed again, and moving existing personally held property into a company can trigger capital gains tax and stamp duty. This guide does not re-argue that decision, because dedicated pages own it. Model the choice on the limited company versus personal ownership tax comparison and read the mechanism in the Section 24 tax relief guide. The finance-relief angle specifically sits on the BTL mortgage tax relief page.
If you land on the company route, the lending detail lives in the existing set of limited-company pages: the limited-company mortgage options, the fuller limited-company buy-to-let guide, and the limited-company mortgage rates market guide. For what a lender actually means by an SPV, which SIC codes they accept and how a brand-new company borrows, see SPV mortgages explained.
Interest-only versus repayment, and why landlords choose interest-only
On an interest-only mortgage you pay only the interest each month and the capital balance stays flat until you repay it at the end, by selling or refinancing. On a repayment (capital and interest) mortgage each payment chips away at the balance, so you own the property outright at the end of the term.
Most landlords take interest-only, for three reasons. First, the lower monthly cost lifts net cash flow, which is the point of a rental business. Second, the lower payment makes the ICR easier to pass, so a given rent supports a larger loan. Third, for a personal landlord the full interest payment is the finance cost that feeds the Section 24 calculation, whereas on a repayment mortgage only the interest slice of each payment counts, and the capital portion is never an allowable expense. Repayment still suits landlords who want to de-gear over time or who are close to exit, but it trades cash flow and borrowing power for a falling balance.
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The lender market: high street, specialist and SPV lenders
There is no single buy-to-let market. Lenders cluster into tiers with very different appetites, which is why the same case can be declined in one place and approved in another:
- High-street lenders. Best rates, tightest criteria. Many prefer straightforward personal borrowers, want two years of accounts or PAYE income, and either do not lend to limited companies at all or only to simple ones. A clean, salaried, personal-name purchase fits here.
- Specialist and challenger lenders. The workhorses of company and portfolio lending. They lend to SPVs, including brand-new ones, take houses in multiple occupation and holiday lets, and accept thinner income evidence, at a modest rate premium.
- Portfolio and private lenders. For landlords with four or more mortgaged properties (the PRA portfolio threshold, defined from 30 September 2017), and for complex or high-value cases that need bespoke underwriting.
A decline is frequently a tier problem, not an affordability problem: the case went to a lender that does not do that kind of business. The types of lender, the criteria that vary between them, and how to tell which tier fits your case are set out in the buy-to-let mortgage lenders guide. Sector context on how much the market lends and to whom is published by UK Finance.
Rates and what drives them
Buy-to-let rates track the Bank of England base rate, but the number you are actually offered is set by a handful of case-specific factors, not a single market figure:
- LTV band. Lower loan-to-value prices better. A 65% loan usually beats a 75% one, which usually beats an 80% one.
- Fix length. Two-year, five-year and longer fixes price differently, and the longer fix is often stress-tested more gently, which can raise how much you borrow.
- Borrower type. Company/SPV lending is typically a small premium over personal borrowing.
- Product fee. A low headline rate often comes with a high arrangement fee. On a small loan the fee can outweigh the rate saving.
- Property type. Houses in multiple occupation and holiday lets sit at higher rates than a standard single let.
We deliberately do not publish a live rate table here, because it would be stale within days. The general rate drivers, the fee-versus-rate trade-off worked through, and how the stress rate interacts with the rate you are offered are covered in the buy-to-let mortgage rates guide, with the company-specific market view in the limited-company rates guide. Whatever figure you see quoted, verify the live market when you apply.
Fees, valuations and the application journey
The rate is only part of the cost. A buy-to-let application typically carries an arrangement or product fee (a flat sum or a percentage of the loan, often added to the balance), a valuation fee, legal and conveyancing costs, and sometimes a broker fee. There can also be telegraphic-transfer or higher-lending charges. Because a percentage-based arrangement fee scales with the loan, on a smaller advance a high fee can cancel out a lower rate, so compare the total cost over the fixed period rather than the headline rate. Arrangement fees are a finance cost for tax, treated on the tax pages rather than here.
The journey itself runs in a familiar order: a decision in principle, then a full application with the rental figure and (for a company) the SPV details, a valuation of the property, an underwriting review against the lender's criteria, a formal offer, and completion through solicitors. The valuation is pivotal for buy-to-let, because the surveyor confirms both the value and the achievable market rent, and it is that rent figure, not your estimate, that the ICR is run against. You can sanity-check the numbers before you start with the buy-to-let mortgage calculator.
Special cases in one place
Most landlords eventually hit a case that a standard single-let template does not cover. Each has its own guide with its own worked example and lender criteria:
- Houses in multiple occupation. Higher rents but specialist valuation, licensing and often an experience requirement. See HMO mortgages.
- Holiday lets. Seasonal income assessed on a blended figure, not the peak week, and the furnished holiday letting tax regime was abolished from April 2025. See holiday-let mortgages.
- Portfolio landlords. Four or more mortgaged properties triggers aggregate stress-testing and a portfolio business plan. See portfolio landlord mortgages.
- Expat and non-resident landlords. A smaller lender panel, higher deposits, and the HMRC non-resident landlord scheme on the rent. Investor cases only; an expat buying a home is regulated. See expat and non-resident landlord mortgages.
- First-time landlords. Fewer lenders, an often-required homeowner status and minimum income, sometimes solved with top-slicing. See first-time landlord mortgages.
- The self-employed. Thin trading history handled because the loan is rental-led, with single-year accounts often accepted. See self-employed buy-to-let mortgages.
- Day-one remortgage. Releasing capital straight after purchase or an SPV transfer, waiving the usual six-month rule. See day-one remortgage for a limited company, and the incorporation-finance detail on how to transfer property into a limited company.
- Brand-new SPV. A company with no accounts still borrows, because the underwriting is on the rent. See SPV mortgages with no income and the SIC code for an SPV property company.
- Capital raising. Pulling equity out of an existing rental by remortgage. See capital raising by BTL remortgage.
One consumer scenario deserves a direct flag. Let-to-buy, where you keep and let your current home and buy a new one, is two mortgages: a buy-to-let on the retained property (business lending we can help with) and a new residential mortgage on the onward home. That onward residential loan is a regulated mortgage contract, and it is not something we introduce. If that is your situation, speak to an FCA-authorised mortgage adviser for the residential leg. The distinction is set out in let-to-buy mortgages.
Where the tax and finance sides fit, and when a broker helps
The two sides of a buy-to-let purchase pull in one direction if you plan them together. The finance question is how to fund the property: the LTV, the ICR, the rate, and which lender tier fits. The tax and structuring question is which wrapper to buy it in: personal name or company, how Section 24 hits your position, and whether an incorporation makes sense. As the worked example showed, the wrapper you choose changes how much you can borrow, so the two decisions are not separable.
A whole-of-market business-finance broker earns their place on the finance side, because the market is fragmented and many specialist and company lenders only accept business through intermediaries. Access to the right lender tier, especially for an SPV or portfolio case, usually runs through a broker rather than direct. What a broker does not decide is the structuring and tax question, which is the property brand's own service.
Our approach is to start with the part that is ours. We can review the SPV and tax side of a purchase (Section 24 exposure, incorporation, deposit efficiency) and, where it is useful, introduce you to a business-finance broker who handles limited-company and portfolio buy-to-let lending. That introduction is a simple hand-off for business-purpose lending, not advice on a specific mortgage product or lender. We do not touch regulated residential finance: where you or a relative will live in the property, that is a regulated mortgage and a matter for an FCA-authorised adviser. This note is general information under the financial promotion rules in section 21 of the Financial Services and Markets Act 2000, not a personal recommendation.