Buy to Let Mortgage Calculator

Use this calculator to estimate your buy to let mortgage affordability. Enter your property details below to check your rental coverage ratio and whether you pass the lender stress tests.

How Buy to Let Mortgage Affordability Works

Buy to let mortgage affordability is calculated differently from residential mortgages. Rather than using income multiples, lenders assess whether the expected rental income from the property is sufficient to cover the mortgage payments with a comfortable margin. This margin is known as the rental coverage ratio, and it is the single most important figure in any buy to let mortgage application.

When you apply for a buy to let mortgage, the lender will instruct a surveyor to provide an independent rental valuation. This figure, not the amount you hope to achieve, is what the lender uses to determine affordability. If the surveyed rent falls short of the required coverage ratio, the maximum loan amount will be reduced accordingly.

Some lenders also consider your personal income as a secondary factor. A minimum personal income of around £25,000 is common, though a handful of lenders have no minimum income requirement at all. The key point is that rental income drives the decision, not your salary.

The 125% and 145% Stress Test Explained

Lenders do not simply check whether the rent covers the mortgage payment at your actual rate. They apply a stress test at a higher notional rate, typically 5.5%, to ensure the property remains affordable if interest rates rise.

The required rental coverage depends on how you own the property and your tax status:

• Limited company: 125% rental coverage at a 5.5% stress rate. Companies can deduct mortgage interest as a business expense, so lenders apply a lower threshold.
• Personal name, basic rate taxpayer: 125% rental coverage at 5.5%. If you pay income tax at the basic rate (20%), most lenders apply the same 125% threshold as limited companies.
• Personal name, higher rate taxpayer: 145% rental coverage at 5.5%. Higher rate taxpayers (40%+) face the strictest requirement because Section 24 tax changes hit them hardest, reducing their net rental income significantly.

This is why your tax position directly affects how much you can borrow. A higher rate taxpayer buying in their personal name needs substantially more rent to qualify for the same mortgage as a basic rate taxpayer or a limited company.

Here is how the calculation works in practice. If you borrow £200,000 at a stress rate of 5.5%, the annual interest is £11,000, which equates to approximately £917 per month. At a 125% coverage ratio, the required rent would be £1,146 per month. At 145%, it rises to £1,330 per month.

If your rental income does not meet the required threshold, the lender will not necessarily decline the application. Instead, they will reduce the maximum loan to a level where the coverage ratio is satisfied. This is why the calculator above is so useful. It lets you test different scenarios before committing to a purchase.

Interest Only vs Repayment for Buy to Let

The vast majority of buy to let mortgages are taken on an interest only basis. This means your monthly payments cover the interest charges only, and the original loan balance remains unchanged throughout the term. At the end of the mortgage, you repay the capital by selling the property, remortgaging, or using other funds.

Interest only payments are significantly lower than repayment, which improves your monthly cash flow and makes it easier to meet the rental coverage requirements. For a £200,000 mortgage at 5%, an interest only payment would be approximately £833 per month, compared to around £1,169 on a 25 year repayment basis.

Some landlords choose repayment mortgages to build equity over time, particularly if they plan to hold the property long term and want it mortgage free by retirement. However, this is a personal financial planning decision rather than a requirement. Your mortgage adviser can help you decide which approach suits your strategy.

Minimum Deposit Requirements

Buy to let mortgages typically require a minimum deposit of 25%, giving a maximum loan to value of 75%. Some specialist lenders offer products at 80% or even 85% LTV, but these come with higher interest rates and stricter affordability criteria.

Your deposit directly affects both the interest rate available to you and the rental coverage calculation. A larger deposit means a smaller mortgage, which means lower monthly payments, which means a better coverage ratio. If you are borderline on affordability, increasing your deposit by even a small amount can make the difference between a pass and a fail on the stress test.

How LTV Affects Your Rate

Lenders price buy to let mortgages in LTV bands. The main thresholds are 60%, 65%, 70%, and 75%. As your LTV decreases, you gain access to lower interest rates. The difference can be significant. A product at 60% LTV might carry a rate 0.5% to 1% lower than the same product at 75% LTV.

This creates a compounding benefit. A lower LTV gives you a lower rate, which reduces your monthly payment, which improves your rental coverage ratio. If you have the capital available, depositing enough to drop into a lower LTV band is one of the most effective ways to improve your overall mortgage terms.

Tax Considerations for Buy to Let Landlords

Since April 2020, individual landlords can no longer deduct mortgage interest as an expense against rental income. Under Section 24 of the Finance Act, mortgage interest relief has been replaced by a basic rate tax credit of 20%. For higher rate and additional rate taxpayers, this has substantially increased the effective tax burden on rental income.

This change is one of the main reasons many landlords now purchase through a special purpose vehicle (limited company). Companies can still deduct mortgage interest as a legitimate business expense before calculating corporation tax, which is charged at 25%. For a higher rate taxpayer, the difference in tax efficiency can be substantial over the life of the investment.

There are trade offs to consider. Purchasing through a company means the property is owned by the company, not you personally. Extracting profits requires dividends, which carry their own tax implications. Mortgage products for limited companies sometimes carry slightly higher rates, although the gap has narrowed considerably as this structure has become more mainstream.

Portfolio Landlord Rules

If you own four or more mortgaged buy to let properties, most lenders classify you as a portfolio landlord. This triggers additional underwriting requirements introduced by the Prudential Regulation Authority in 2017.

As a portfolio landlord, you will need to provide a full portfolio schedule showing every property you own, the outstanding mortgage on each, the current rental income, and the current value. The lender will assess the overall health of your portfolio, not just the individual property you are applying for. They will look at the aggregate LTV, the aggregate rental coverage, and your overall borrowing exposure.

Not all lenders apply these rules in the same way. Some are more restrictive, while others specialise in working with experienced portfolio landlords. A whole of market mortgage adviser can identify which lenders are most suitable for your specific portfolio, saving considerable time and avoiding unnecessary credit searches. If you are a self employed landlord, specialist advice is especially valuable as some lenders have more flexible income verification requirements.

Tips to Improve Your Rental Coverage

If the calculator shows your rental coverage is falling short, there are several strategies to consider.

First, increase your deposit. Even a modest increase can reduce your mortgage amount enough to bring the coverage ratio above the required threshold. Second, consider purchasing through a limited company if you are a higher rate taxpayer. The lower 125% stress test requirement gives you more headroom compared to the 145% required for individual ownership.

Third, review the rental market carefully. Furnished properties in high demand areas often command higher rents. Small improvements such as adding a second bathroom or upgrading the kitchen can justify a meaningful rent increase. Fourth, consider whether the property type is optimal. Houses in multiple occupation (HMOs) generate higher rental yields than standard single lets, though they come with additional licensing and management requirements.

Finally, shop around. Different lenders use different stress rates and coverage ratios. Some lenders apply a 5% stress rate rather than 5.5%, and a few use actual pay rate calculations for certain products. A whole of market broker has access to these variations and can match you with the most favourable criteria for your situation.

Frequently Asked Questions

What is a good rental coverage ratio?

Most lenders require a minimum of 125% for limited company purchases and 145% for individual purchases, both calculated at a stress rate of 5.5%. A higher ratio gives you a larger buffer and may give you access to a wider range of lenders and products.

Can I get a buy to let mortgage with a 20% deposit?

A small number of lenders offer buy to let mortgages at 80% LTV, which requires a 20% deposit. However, the majority of the market requires 25% or more. Products at 80% LTV typically carry higher rates and tighter affordability criteria.

Do I need a minimum income for a buy to let mortgage?

Most lenders require a minimum personal income of around £25,000 per year. Some lenders have no minimum income requirement, particularly for experienced landlords or those purchasing through a limited company.

Is interest only the best option for buy to let?

Interest only is the most common choice because it maximises monthly cash flow and makes it easier to meet rental coverage requirements. However, some landlords prefer repayment mortgages to build equity over time. The right choice depends on your investment strategy and financial goals.

Should I buy in my personal name or through a limited company?

It depends on your tax rate. If you are a basic rate taxpayer, the stress test is 125% either way, so the decision comes down to long-term tax planning. If you are a higher rate taxpayer, a limited company reduces the stress test from 145% to 125%, making it significantly easier to qualify. There are costs involved in setting up and running a company (accounting, filing, dividend extraction), so the decision should be based on your individual circumstances and the number of properties you plan to hold.

How does the stress test work if rates are already above 5.5%?

If your actual mortgage rate is above the standard stress rate, some lenders will stress test at the actual rate instead. Others maintain their standard stress rate regardless. Your mortgage adviser can confirm which approach each lender takes.

Can I use rental income from other properties to support my application?

Some lenders allow surplus rental income from your existing portfolio to be used as supporting income for a new application. This is known as cross collateralisation or top slicing. It is more common with portfolio landlord specialists and can be useful when a specific property falls slightly short on rental coverage.

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