How the model works

You have no reason to take someone else's arithmetic on trust. This page is the whole method, and the tables further down are read straight out of the code that works out your result, so they cannot quietly disagree with it. If you spot something wrong, there is a form at the bottom of the page.

The three limits

Three separate things can stop you, and the model works out all three rather than collapsing them into one number.

1. Your monthly cash flow. Take-home pay, less what you actually spend, less the extra running costs of a larger home, less the saving you intend to protect and the buffer you would rather not touch. Whatever remains is what a payment can consume.
2. The lender's income multiple. A fixed multiple of gross household income. Real lender assessments also deduct childcare, credit commitments and pension contributions first, and stress-test above the pay rate, so a household that looks fine on the multiple can still be declined.
3. Your deposit. Lenders advance up to a share of the price. Once the deposit will not carry the loan to value, nothing else matters.

The answer is the lowest of the three, and the result always names which one it was. A price with no limiting factor attached invites the wrong conclusion: the person stopped by a deposit and the person stopped by an income multiple need to do completely different things next.

Why saving is an input, not a leftover

Most calculators treat saving as whatever happens to remain. A model built that way can only ever conclude that the house is affordable, because there is always something left once you stop saving. Here the saving is subtracted before the budget is worked out, and the result reports the saving that actually survives at the price you land on — not the one the scenario assumed to get there.

That distinction matters more than it sounds. If the loan is capped by income or deposit, money freed up by cutting the saving is never spent. Reporting the cut as though it happened makes an expensive scenario look like it leaves more money over than a cheap one, which is impossible.

The circular bits

Purchase tax comes out of the same cash that forms the deposit, and it depends on the price that deposit is buying: price = loan + cash − tax(price). There is no clean way to solve that in one step, so the model iterates until the figure stops moving, which takes a handful of passes.

The interest rate is circular too — it depends on the loan to value, which depends on the loan, which depends on the rate. Same treatment: repeat until the rate settles.

Porting

A ported mortgage is not one loan at a blended rate. It is two: the balance carried across at the old rate, and the top-up at today's. The model treats them separately, which is the single largest lever in it and the thing generic calculators leave out.

Two caveats the result repeats. A lender will usually port the rate but assess the top-up on its own terms, and can decline that part. And the ported rate is temporary — when the deal ends the whole balance moves onto one new rate together.

When your fixed rate ends

A fixed rate is a few years long and a mortgage is decades long, so the payment you start on is not the payment you live with. When the fix runs out you take another one, at whatever rates are by then — very few people let the deal lapse onto the lender's standard rate, which is why the model does not assume they do.

So you give it a rate to assume for that next deal, and it applies that rate to the whole balance for the rest of the term. That figure is shown in the same size type as the headline payment, because showing you only the opening payment would be leaving out the part that changes what a house costs you.

The default sits above today's rates on purpose. It is not a forecast, and nobody has one — it is a test of whether the house still works if borrowing is dearer when the deal comes round. Put your own figure in and watch what moves.

Take-home pay

You are asked for gross salary and actual take-home rather than for a list of deductions. The difference between statutory take-home and the figure you give is held as a single ratio and applied as the salary moves. It is accurate close to where you calibrated it and degrades with distance, which is fine for "what if this salary dropped ten per cent" and wrong for "what if I earned three times as much".

Student loan repayments are the weak point: they start at a threshold rather than scaling smoothly, so a single ratio over- or under-states them either side of it.

What the tables say today

What this is not

It is not advice and not a personal recommendation. It describes what happens to figures you enter. It names no lender and no product, takes no referral fee, and has no reason to want your number to be larger than it is.

It is also not a mortgage illustration. Lenders apply their own affordability assessment and may lend more or less. Your home may be repossessed if you do not keep up repayments on a mortgage secured against it.