Introduction
One of the most common questions for homebuyers is: How much can I borrow on a mortgage?
The answer depends on the lender’s affordability assessment, a detailed calculation designed to evaluate how much you can realistically repay each month without financial stress.
Mortgage affordability assessments have become increasingly sophisticated, using income, expenditure, credit behaviour, future interest rate risks, and lifestyle costs to determine safe loan levels.
This guide explains exactly how lenders calculate affordability – and what you can do to maximise your borrowing power.
What Is a Mortgage Affordability Assessment?
Affordability is an evaluation of your ability to repay your mortgage both now and if interest rates rise.
Unlike older lending models that used simple income multiples, today’s assessments consider your full financial picture, including:
- Income
- Regular expenses
- Existing credit commitments
- Future financial obligations
- Interest rate stress-testing
- Household composition
The assessment varies by lender, which is why two lenders can offer very different borrowing amounts.
Step 1: Analysing Your Income
Lenders start with your total gross income. This includes:
- Employment Income
Basic salary, overtime, bonuses (often at 50–75%), and commission.
- Self-Employed Income
Averaged over 2–3 years based on SA302s or company accounts.
- Additional Income
- Benefits
- Child maintenance
- Rental income
- Car allowance
- Investments/dividends
Lenders differ significantly in how much of each income type they accept.
Step 2: Assessing Your Outgoings
Lenders deduct essential monthly outgoings, including:
- Credit card payments
- Loans and finance agreements
- Car payments
- Household bills
- Council tax
- Childcare costs
- School fees
- Pension contributions
- Travel expenses
Some lenders also look at discretionary spending to ensure repayment reliability.
Step 3: Reviewing Your Credit Commitments
Debt has a substantial impact on affordability. For example:
- A £300/month car finance payment could reduce borrowing by £15,000–£20,000.
- High utilisation on credit cards can reduce the amount offered.
Lenders want to see responsible credit use and minimal unsecured debt.
Step 4: Household Composition
A single applicant earning £40,000 will be assessed differently to a family of four on the same income.
More dependants = higher living costs = reduced borrowing.
Step 5: Stress-Testing Future Interest Rates
To protect borrowers, lenders must test affordability at higher future interest rates.
Typically, they stress-test at rates 2–3% above the current deal.
If you fail the stress test, your borrowing amount will be reduced.
Step 6: Calculating a Maximum Loan Amount
After income and expenses are analysed, lenders use a formula to calculate the maximum safe loan amount.
This usually results in a borrowing range of 4 to 5.5 times your income, depending on:
- Credit score
- Deposit size
- Lender policy
- Employment type
- Dependants
- Existing debts
Borrowers with strong financial profiles may reach the upper end of the range.
How Joint Applications Are Assessed
Both incomes are combined, but household expenses also increase.
A couple with strong income and low debts can borrow substantially more than a single applicant.
Why Affordability Varies Between Lenders
Different lenders use different algorithms, income acceptance rules, and risk appetites.
This is why:
- One lender may offer £220,000
- Another may offer £275,000 on the same income
A mortgage broker can identify which lenders are most generous for your circumstances.
Factors That Reduce Affordability
- High monthly credit repayments
- Low credit score
- Irregular income
- High childcare costs
- Multiple dependants
- Recent credit issues
- Large car finance agreements
Each of these reduces the maximum mortgage available.
How to Increase Your Borrowing Power
- Reduce unsecured debt
- Pay off car finance if possible
- Improve your credit score
- Increase your deposit
- Extend your mortgage term
- Opt for a lower-rate deal
- Add a second applicant
- Use a specialist lender
Improving affordability even slightly can increase borrowing capacity significantly.
Conclusion
Mortgage affordability assessments are detailed and tailored to your individual financial situation.
By understanding how lenders calculate affordability – and taking steps to improve your financial profile – you can maximise your borrowing potential and achieve a smoother, more successful mortgage application.
YOUR HOME MAY BE REPOSSESSED IF YOU DO NOT KEEP UP REPAYMENTS ON YOUR MORTGAGE.
Bransgroves Mortgage Brokers is a trading name of Just Mortgages Direct Limited which is an appointed representative of The Openwork Partnership, a trading style of Openwork Limited which is authorised and regulated by the Financial Conduct Authority.
Approved by The Openwork Partnership on 03/12/2025
