Mortgage Affordability Explained: How Lenders Decide What You Can Borrow

There is a big difference between what you can borrow and what you can comfortably afford, and an equally big difference between one lender and the next.

Two lenders can look at exactly the same payslips and arrive at loan figures tens of thousands of pounds apart. Understanding why is the key to working out how much you can borrow, and to borrowing well.

What affects mortgage affordability

Affordability is not simply a multiple of your salary. It is a lender’s judgement about whether you can keep up your mortgage payments comfortably, month after month, alongside everything else you have to pay for, such as existing credit commitments, household bills, childcare, and everyday living costs.

In other words, how much mortgage you can afford is about far more than income alone.

Eligibility and affordability are not the same thing either.

You might tick every box for a particular product, but the amount a lender is prepared to advance still comes down to its own affordability assessment. The sensible starting point is always the same: you should never borrow more than you can comfortably afford to repay, both now and if your circumstances change.

Why lenders reach different answers

This is the part most guides skip, and it is the part that matters most when you are asking how much you can borrow based on your salary.

Every lender builds its own affordability model. They share the same regulatory framework, but within it they make very different choices about income and spending.

That is why speaking to the right lender for your situation can make such a difference.

A few of the ways lenders differ in practice, and some of the main things that affect mortgage affordability:

• The affordability model itself. Rather than relying on a simple salary multiple, most lenders now run your income against your outgoings and commitments to work out how much you can realistically afford each month. Two lenders can plug in the same figures and reach very different maximum loans, because their models weigh things up differently.
• How they treat different income. Basic salary, bonus, commission, overtime, self-employed profit, and second jobs are all treated differently from lender to lender, from using none of a given source to using all of it.
• Pension contributions. Where you pay into a pension by salary sacrifice, some lenders assess you on the reduced salary that shows on your payslip, while others will add those voluntary contributions back and lend against the higher figure. If you make meaningful voluntary or additional pension contributions, choosing a lender that adds them back can materially increase your borrowing.
• Committed expenditure and background debt. Lenders weight loans, credit cards, car finance and childcare differently, so the same commitments can reduce your borrowing more with one lender than another.

Because these choices vary so widely, the right lender for your circumstances is often worth far more than a small difference in headline rate. This is exactly where a whole-of-market broker earns their keep.

How do lenders calculate affordability?

Income

Income is the starting point, but it is only the starting point. Lenders take your income and then feed it through their own affordability model alongside your outgoings and commitments to arrive at a figure. What counts as income, and how much of each type they will use, varies from lender to lender, so the same salary can support very different loans.

Spending

Lenders look closely at your outgoings to work out how much is genuinely left over each month. Regular commitments, bills, and lifestyle spending all feature, so it is worth having your accounts in good order in the months before you apply.

Children and dependants

The number of children or other financial dependants you have directly affects how much a lender will lend. More dependants mean higher assumed living costs, which reduces the income left over for a mortgage. Lenders factor this into every affordability calculation, so two identical salaries can support quite different loans depending on family circumstances.

Maintenance and other regular commitments

Regular payments such as child maintenance, spousal maintenance, and school fees are treated as committed expenditure and reduce the amount you can borrow, in much the same way as a loan or credit commitment. Equally, where you receive maintenance, some lenders will count it as income if it is well-evidenced and expected to continue. As with everything else, lenders differ, so it is worth matching your circumstances to a lender that treats them favourably.

Debt-to-income

Your existing debts relative to your income matter. The lower the proportion of your income already going on repayments, the more room there is for a mortgage.

Ways to improve your affordability

Some of these are quick wins, others take a little planning.

A broker can tell you which will make the biggest difference for you, and many people find a mortgage affordability calculator a useful starting point before getting tailored advice.

• Match yourself to the right lender. Often the single most effective step. The lender that treats your income and outgoings most generously can increase your borrowing without you changing anything about your finances.
• Reduce your existing debts. Clearing or lowering credit cards, loans, and car finance frees up income and improves your debt-to-income position.
• Keep your credit profile healthy. Register on the electoral roll, keep payments up to date, and check your credit report for errors before you apply.
• Save a larger deposit. Borrowing less relative to the property value both improves affordability and tends to unlock better rates.
• Tidy up regular spending. Trimming non-essential outgoings in the run-up to an application can leave more disposable income for a lender to work with.
• Consider the mortgage term. A longer term lowers monthly payments and can improve affordability, though you will usually pay more interest overall, so it is a balance.
• Buy jointly. Combining incomes with a partner or co-buyer on a joint mortgage usually increases the total you can borrow.
• Explore a Joint Borrower Sole Proprietor (JBSP) arrangement. A JBSP mortgage lets a family member (often a parent), or in some instances a friend, add their income to the affordability assessment to help you borrow more, while you remain the sole legal owner of the property. It can be a very effective way to boost borrowing without putting the supporting person on the title deeds. It is not right for everyone, and there are tax and legal points to weigh up, so it is worth talking through properly.
• Look at your pension contributions. If you make voluntary or salary-sacrifice pension contributions, the right lender may add these back and lend against your higher income.
• Use a whole-of-market broker. A broker can identify the lenders whose affordability model treats your particular circumstances most generously, and structure the application to present you at your best.

A word on borrowing responsibly

Stretching to the absolute maximum is not always the wise choice.

Rates, bills and life all change, so leaving yourself some breathing room is sensible. The goal is not just to get a mortgage, but to hold one comfortably. If money ever gets tight, speak to your lender early, as they would far rather work with you than see payments missed.