Buying a House: The Numbers That Actually Decide What You Can Afford

The answer, up front. Affordability isn't the price a mortgage broker will lend you — it's the monthly total cost of ownership you can carry without starving every other goal, plus the one-off cash you'll never see again (stamp duty, legals, moving) that most buyers under-budget by thousands. The buyers who do this well decide their number from their budget first, use the Lifetime ISA and its 25% bonus years before they need it, and time the purchase around the stamp duty thresholds that changed in April 2025. The ones who don't get "approved" for a payment that quietly cancels their pension, their holidays and their buffer for a decade.

This is educational information, not regulated financial advice or a personal recommendation.

Buying a home is often the largest transaction of someone's life and the one they plan the least — they plan the house, not the finances. Here's the order that keeps you solvent and sane.

The one idea most people miss

The mortgage a lender offers you is a measure of their risk, not your life. It's calculated on affordability rules, not on the pension you want to keep funding, the childcare that's coming, or the holidays that make life worth living. Start from your budget, not the maximum loan. Decide the monthly housing cost you can carry while still funding everything else, then work backwards to a price — not the other way round.

A decision framework for the purchase

1. Budget the total monthly cost of ownership — not just the mortgage.
The mortgage payment is the headline; the real cost includes buildings insurance, service charge/ground rent (on leaseholds — these can be brutal), council tax, maintenance (budget ~1% of the property value a year), and higher energy bills on a larger place. A £1,400 mortgage can be a £1,900 monthly reality. Model the whole number against your budget before you fall in love with anything.

2. Size the deposit — and the LISA — early.
A bigger deposit unlocks lower loan-to-value bands and cheaper rates; the jumps at 90%, 85%, 80% and 75% LTV are real money. The under-used tool: the Lifetime ISA (LISA). You can pay in up to £4,000 a year (within your £20,000 ISA allowance) and the government adds a 25% bonus — up to £1,000 a year of free money — usable for a first home up to £450,000. The catch to plan around: you must have held it 12 months before use, and withdrawing for anything other than a first home or age 60 costs a 25% penalty (which claws back more than the bonus). Open one early, even with £1, to start the clock.

3. Budget the one-off cash you never see again.
Beyond the deposit, the transaction itself costs real money that can't come from the mortgage: Stamp Duty Land Tax (SDLT), solicitor/conveyancing fees, survey, mortgage arrangement fee, and moving costs. From 1 April 2025 the SDLT thresholds tightened:

• Standard residential: 0% to £125,000, then 2% to £250,000, 5% to £925,000, 10% to £1.5m, 12% above.
• First-time buyers: 0% up to £300,000, 5% on £300,001–£500,000, and no relief at all above £500,000.
• Additional property (second home / buy-to-let): a 5% surcharge on top of standard rates.

4. Mind the thresholds — a few thousand over a line costs disproportionately.
Because rates step up at thresholds, buying just above one (especially the £500,000 first-time-buyer cliff, where relief vanishes entirely) can cost thousands more than a slightly cheaper home. This is a genuine negotiation lever: a price of £505,000 vs £500,000 isn't a £5,000 difference to a first-time buyer — it can be far more once relief is lost.

5. Choose the mortgage structure deliberately.
Fixed vs variable, and the term, are cashflow decisions, not just rate decisions. A longer term lowers the monthly payment but raises lifetime interest; a shorter one does the reverse. Stress-test your payment against a rate a few points higher — the question isn't "can I afford this rate," it's "can I afford the renewal."

Decide your number from your budget. Connect your finances and map it in the aggviz planner: model the total monthly cost including service charge and maintenance, the one-off SDLT-and-fees cash, and what's left for pensions and life — free, on data you own.

A worked mini-example

Tom and Aisha, first-time buyers, are looking at a £480,000 flat and a £505,000 flat.

• SDLT on £480,000 (first-time relief): 0% on the first £300,000, 5% on £180,000 = £9,000.
• SDLT on £505,000: above £500,000 first-time relief is lost entirely, so standard rates apply: 0% to £125k, 2% to £250k (£2,500), 5% to £505k (£12,750) = £15,250.
• The £25,000 higher asking price actually costs them ~£31,250 more once the lost relief is counted — before legals and the bigger deposit.

Separately, they realise each had opened a LISA two years earlier: £8,000 each contributed, plus £2,000 each of bonus = £4,000 of free money toward the deposit, simply for starting the clock early.

The house didn't change. Understanding the thresholds and using the wrapper changed their outcome by tens of thousands.

Common mistakes

• Borrowing the maximum and quietly cancelling every other goal to service it.
• Budgeting the mortgage only — forgetting service charge, maintenance, insurance, council tax.
• Buying just over an SDLT threshold, especially the £500k first-time-buyer cliff.
• Not opening a LISA early — losing the bonus and tripping the 12-month rule.
• Under-budgeting the one-off cash (SDLT + legals + survey + moving).
• Fixing to the lowest rate without stress-testing the renewal.

A short pre-purchase checklist

• Set your price from your budget, not the maximum loan.
• Model the total monthly cost (mortgage + service charge + maintenance + insurance + council tax).
• Open a LISA early; use the 25% bonus if buying under £450,000.
• Budget the one-off cash: SDLT, legals, survey, arrangement fee, moving.
• Check the SDLT thresholds — negotiate around the lines.
• Stress-test the payment against a higher renewal rate.

A house you can only just afford owns you. A house sized to your budget leaves room for the rest of your life to keep happening.

Related reading: When your savings cross a threshold · The cost-of-living squeeze · Moving country: cross-border money moves

Do it next

1. Model it yourself, free. Map the purchase in the aggviz planner — total monthly cost, SDLT and fees, what's left for everything else — on data you own.
2. Want it looked at properly? Join the advice waitlist and a real person will call you when the time's right. Human, regulated, never automated.

This is educational information, not regulated financial advice or a personal recommendation. aggviz provides planning tools and education on data you own; SDLT rates and mortgage terms depend on your circumstances and change — confirm the current figures and your position with a conveyancer, mortgage broker, or qualified adviser before acting.

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