Mortgage Affordability Calculator

What "mortgage affordability" actually means

There are three different numbers behind "how much mortgage can I afford?". The lender's number (what they'll approve). The regulator's stress-tested number (what the lender verifies you could still afford at higher rates). Your personal number (what you can afford without sacrificing quality of life or emergency buffer). Most people only know the first. The gap between the three is where affordability mistakes happen. This calculator handles the lender's number; the commentary below is about the other two.

The lender's core equation

Mortgage lenders typically approve loans up to 4-4.5× gross household income, sometimes 5× for strong applicants. A couple earning 80,000 combined can borrow roughly 320,000-400,000. Add the deposit to reach the purchase price: 15% deposit on 380,000 = 57,000 deposit, so 447,000 property buying power. These are the surface calculations. Beneath them, lenders apply more detailed affordability models that consider existing debts, childcare costs, and commitment levels — which is why two households with identical gross incomes can receive different approvals.

The mandatory stress test

Post-2008, regulation requires lenders to verify that borrowers could still afford the mortgage at rates 3 percentage points higher than the offered rate. If your fixed rate is 4.5%, the stress test confirms you could afford payments at 7.5%. This isn't theatre — it's the regulator's memory of 2006-era affordability calculations that assumed rates would stay low forever. If you're offered a 4.5% rate but the payment at 7.5% would be untenable, you've been approved for more than you might borrow. Lenders will still approve; the stress test filters some but not all such cases.

The 28/36 personal affordability filter

The industry-standard personal affordability rule: no more than 28% of gross monthly income to housing costs (mortgage + insurance + local property tax + maintenance), and no more than 36% to total debt obligations (housing plus credit cards, car loans, student loans). On 80,000 gross household income (6,667/month gross), that's 1,867/month max housing and 2,400/month max total debt. Numbers above these levels aren't automatic failures but tend to produce financial stress. Numbers below them create flexibility for life events. Most post-mortgage regret stories involve borrowers who ignored the 28% filter.

The three income multiplier tiers

Mortgage income multipliers vary by applicant strength:

Standard (4.0-4.5×): Default for most applicants. Clean credit, stable income, typical debt levels.

Enhanced (4.5-5×): Available to professionals with stable income (doctors, lawyers, accountants), high earners (75,000+), or applicants with very low existing debt.

Specialist (5-6×): Rare but possible for applicants with exceptional circumstances — guaranteed future income, significant liquid assets, or specific lender-professional relationships.

Pushing toward the upper end of available multipliers is only useful if you can still afford payments at the 3%-higher stress-tested rate. Borrowing at 5× income at the top of your band leaves little room for rate rises at the next remortgage.

The affordability costs lenders don't count

Mortgage affordability calculations include the mortgage payment itself but often miss:

local property tax: 1,500-3,000/year depending on area and band.

Buildings insurance: 200-500/year. Required by mortgage lenders.

Maintenance reserve: 1% of property value annually is the industry rule of thumb. On a 400,000 property: 4,000/year or 333/month — not optional and not going away.

Service charges: 1,000-3,000/year for flats, sometimes more. Not included in freehold ownership but sneaky additions in new-build or managed properties.

Transaction costs on move: Buying costs 3-5% (Stamp Duty, legal, survey). Future selling costs another 1-3%. Usually paid from savings, not mortgage, but affect overall capacity.

Total additional housing costs: 500-1,000/month on typical properties. Lender affordability models include some but not all; personal affordability math should include everything.

The deposit size leverage

Deposit affects affordability in two ways beyond the obvious smaller loan. First, LTV-tiered rates: 95% LTV typically costs 1-2 percentage points more than 75% LTV. On a 400,000 property, 10% vs 20% deposit can mean 200-300 more per month at the same fixed rate. Second, lender scrutiny: higher LTV loans receive closer affordability verification. Applicants stretching on both LTV and income multiplier face tougher approval than those stretching on just one. Waiting to build deposit rather than stretching on income is the more sustainable path for borderline applicants.

The self-employed affordability disadvantage

Self-employed applicants face systematically tougher affordability assessments:

Two years of accounts minimum (some lenders require three).
Net profit (not revenue) used for affordability math — often (commonly cited at 30-50%) lower than salaried equivalent.
Income averaged across years, penalising growth periods.
Higher buffers applied to account for income variability.

A self-employed consultant earning 80,000 profit can typically borrow (commonly cited at 20-30%) less than a salaried professional with the same 80,000 gross. This affects when it's economically rational to go self-employed and when it's worth staying salaried through a house purchase before transitioning. Some specialist lenders (Kensington, Pepper) take more nuanced views of self-employed income and are worth noting if mainstream lenders reject.

What gets you to a "yes"

The main factors lender affordability models weight most:

Income stability (salaried permanent beats contract beats self-employed).
Deposit size (20%+ is strongly preferred over 10-15%).
Clean credit (no defaults, low utilisation, some history).
Low existing debt service (credit cards paid down, car loans minimal).
Reasonable loan-to-income ratio (4-4.5× is comfortable; 5× is stretch).
Evidence of savings discipline (deposit accumulated over time beats gifted).

Moving each factor toward the favourable end increases the available borrowing and available rates. Stretching on one while others are strong usually works; stretching on multiple simultaneously often doesn't.

What this calculator shows

The tool estimates borrowing capacity based on income and deposit. It doesn't automatically run the 28/36 personal affordability filter, stress test at +3%, or include housing ownership costs. Use the figure as the lender's likely maximum; apply the other filters to determine what you might actually borrow.