How Much Mortgage Can I Afford? Affordability Rules and Real Calculations

How lenders calculate how much to lend

Most lenders use an income multiple as a starting point: typically 4 to 4.5 times your gross annual income for a single application, or 4 times combined income for a joint application. Some lenders offer up to 5 or 5.5 times income for applicants with strong profiles (high income, large deposit, low outgoings). This gives a rough upper limit before the detailed affordability assessment begins.

The detailed assessment then checks your monthly outgoings: existing debt repayments, credit card minimums, car finance, childcare costs, and committed spending. These are deducted from your net monthly income and the remaining 'disposable' income is assessed against the proposed mortgage payment at a stressed interest rate - typically 2-3 percentage points above the current rate - to test whether you could afford repayments if rates rose.

The 28 and 36 rules

US mortgage guidelines traditionally use two ratios. The front-end ratio: your monthly mortgage payment (principal, interest, taxes, and insurance) should not exceed 28% of your gross monthly income. The back-end ratio: all monthly debt payments combined (mortgage plus car loans, student loans, credit cards) should not exceed 36% of gross monthly income. Lenders may approve loans that breach these thresholds with compensating factors such as a large deposit or high credit score, but they remain useful personal guidelines.

Example: gross monthly income 5,000. Front-end limit (28%): 1,400 maximum mortgage payment. Back-end limit (36%): 1,800 total debt payments. If you already pay 400 per month on a car loan, your maximum mortgage payment under the back-end rule is 1,800 - 400 = 1,400. In this case both rules arrive at the same number - but with higher existing debts, the back-end rule would be the binding constraint.

What deposit do you need?

The minimum deposit for most residential mortgages is 5-10% of the purchase price, but the interest rate you pay falls significantly with a larger deposit. The pricing steps typically occur at 5%, 10%, 15%, 20%, 25%, and 40% deposit. The difference in monthly payment between a 5% deposit mortgage and a 25% deposit mortgage on the same property can easily be 300-500 per month due to both the larger loan and the higher interest rate at low deposit levels.

A larger deposit also reduces your loan-to-value (LTV) ratio, which affects both approval likelihood and rate. At 95% LTV (5% deposit) you are in the highest-risk tier. At 75% LTV (25% deposit) you access most standard product rates. Below 60% LTV you reach the lowest rates available.

The impact of interest rates on affordability

A 1 percentage point rise in interest rate on a 300,000 mortgage over 25 years increases the monthly payment by approximately 150-170. A 2 point rise adds 300-350 per month. This is why lenders stress-test applications at rates above the current level - to ensure the loan remains affordable if rates rise. When planning your budget, model payments at your current rate and at 3 percentage points higher to understand your exposure.

Hidden costs most buyers underestimate

• Stamp duty or transfer tax - in the UK, stamp duty land tax (SDLT) on a 350,000 property is 7,500 for a first-time buyer (as of 2026). In the US, transfer taxes vary significantly by state.
• Legal fees (conveyancing) - typically 1,500-3,000 in the UK. Budget 1,000-2,500 in the US for a real estate attorney where required.
• Survey and valuation fees - a basic valuation is often included by the lender; a full structural survey costs 600-1,500 depending on the property.
• Mortgage arrangement fee - many fixed-rate products charge 999-2,000 to arrange. This can be added to the mortgage but then accrues interest over the full term.
• Moving costs - professional removals for a 3-bedroom house typically cost 1,000-2,500 for a local move.
• Initial repairs and furnishing - budget at least 5-10% of the purchase price for the first year of ownership for repairs, improvements, and furnishings.

Common mistakes

• Borrowing the maximum the lender offers - the maximum is not the same as comfortable. Model your budget at the maximum mortgage payment and decide whether you are genuinely happy with what remains.
• Ignoring the stress test rate - monthly payments at current rates feel manageable until they do not. Always check affordability at current rate plus 3%.
• Forgetting ongoing costs - council tax, buildings insurance, service charges (for leasehold), and maintenance commitments are real monthly costs that renters often underestimate.
• Not checking your credit file before applying - errors on your credit record can cause a rejection even with a good income. Check all three major agencies (Equifax, Experian, TransUnion) at least 3 months before applying.

Use the Mortgage Affordability Calculator

The Mortgage Affordability Calculator on this site lets you enter your income, deposit, interest rate, and term to see your maximum borrowing, estimated monthly payment, and total interest cost. Adjust the rate slider to stress-test your affordability at higher rates. It is an estimate - lenders will apply their own detailed assessment - but it gives you a realistic starting range before you speak to a broker or lender.