What a house sale what-if is actually modelling
Moving house is three transactions wearing one coat: you sell a property, you redeem the mortgage secured on it, and you buy something else with whatever survives. The figure that decides whether a move works is not the sale price, and not the asking price of the next place. It is the equity that reaches the other side, after the loan and the costs have taken their share.
This calculator works through that in order. Sale price, minus the mortgage still outstanding, minus any early repayment charge for settling it, gives the equity released. You then choose how much of that equity to put down as a deposit; the rest is the cash you keep. The new mortgage is simply the price of the new property less the deposit, and the monthly payment follows from the rate and term you enter. The remaining fees are subtracted from the cash you kept back, which is what the final cash-after-fees figure represents.
That ordering matters. If you put every last pound of equity into the deposit, the fees still have to come from somewhere, and the cash-after-fees figure goes negative to tell you so. A negative number there is not a rounding artefact. It is the amount you would need to find from savings to complete.
A worked example
Selling at £350,000 with £220,000 still outstanding and a £3,300 early repayment charge leaves £126,700 of equity. Put £50,000 of that down on a £400,000 purchase and you keep £76,700 in cash, with a new mortgage of £350,000. That is a 12.5% deposit, costing about £1,945 a month over 25 years at 4.5%.
Then the costs of moving:
| Cost | Example |
|---|---|
| Estate agent, 1.2% + VAT on £350,000 | £5,040 |
| Conveyancing on the sale | £1,200 |
| Stamp duty on a £400,000 purchase | £10,000 |
| Conveyancing on the purchase | £1,500 |
| Removals | £1,200 |
| Everything else | £500 |
| Total | £19,440 |
So £76,700 of retained cash becomes £57,260. The alternative, putting the whole £126,700 into the deposit, cuts the mortgage to £273,300 and the payment to about £1,519 a month, a saving of £426 every month, but leaves nothing for the £19,440 of fees. Comparing those two runs is the point of the tool: the same sale supports quite different moves, and the trade is almost always monthly payment against cash in hand.
The costs people underestimate
- Estate agent fees. Quoted as a percentage of the sale price, and the quote may or may not include VAT at 20%. A "1%" fee is 1.2% if VAT is extra, which on a £350,000 sale is £700 of difference. Check which you were quoted before you enter it.
- Stamp duty. Payable on the purchase, in cash, on completion, and it cannot be added to the mortgage. Rates are banded, differ for first-time buyers, and differ again in Scotland (Land and Buildings Transaction Tax) and Wales (Land Transaction Tax). If you will own a second property at completion, the additional-property surcharge applies on top. Use the official calculator for your nation and enter the result rather than estimating it.
- Conveyancing, twice. One firm handles both sides, but there are two sets of fees and two sets of disbursements: searches, Land Registry, bank transfers. Ask for a quote that includes disbursements, not just the legal fee.
- The early repayment charge on the mortgage you are leaving. If you are inside a fixed period, settling the loan early usually triggers a charge of a few percent of the balance. Enter it in the sale section; it comes off the equity before anything else.
- Survey, EPC, removals and the long tail. A homebuyer's report, the EPC the sale legally needs, a removals firm, storage if the dates do not line up, and the locks, curtains and white goods that always follow a move.
Porting or redeeming
Most UK mortgages are portable: rather than settling the loan and taking a new one, you move the existing deal to the new property and usually avoid the early repayment charge. It is worth establishing which route you are on before modelling anything, because it changes the arithmetic at the top.
Porting is not as clean as it sounds. You still have to pass the lender's current affordability checks, the new property still has to be acceptable security, and if you are borrowing more the extra comes as a second part at today's rate, leaving you with two sub-accounts on different rates, ending on different dates. If you are porting, set the early repayment charge to zero and treat the rate you enter as a blend. If you are redeeming, enter the charge and the rate you have actually been offered.
Why the range matters more than the number
Every input here is an estimate, and the two largest are the least certain: what your property actually sells for, and what you actually pay for the next one. Both are negotiated, and both can move by more than all the fees combined.
The useful discipline is to run three scenarios rather than one. Take a sale £10,000 under your asking price with a purchase at full price, then the midpoint, then the optimistic case, and see whether the move still works at the bottom of that range. A plan that only survives the best case is a plan that depends on a chain of strangers behaving conveniently. Timing carries the same risk: if the sale completes before the purchase you may need bridging or temporary accommodation, both of which cost money the model does not know about.
Common questions
What if I am in negative equity?
If the mortgage outstanding exceeds the sale price, the equity figure goes negative and the tool shows a shortfall: that is money you must find on completion, because the loan has to be cleared before the property can be transferred. The realistic options are to wait for the balance to fall or values to recover, to make overpayments to close the gap using the overpayment calculator, or to ask your lender about a negative equity mortgage, which a few offer to existing borrowers moving home. Take advice before committing to a sale in this position.
Do I pay capital gains tax on my own home?
Usually not. Private residence relief normally exempts the sale of your only or main home. It can be restricted if you let the property out, used part of it exclusively for business, or the grounds exceed the permitted area, and it does not apply to a second home or a buy-to-let. If any of that applies, check the current rules or ask an accountant. The exemption is the default, not a guarantee.
How accurate is the monthly payment?
It is a standard repayment calculation on the new loan at the rate and term you enter, and it excludes insurance, service charges and any product fee added to the loan. For more detail on how the payment is derived and what shifts it, see the mortgage calculator.
Figures on this page are illustrative and rounded, and tax rates and thresholds change. This is general information, not financial or tax advice; confirm redemption figures, early repayment charges and stamp duty with your lender, solicitor or a qualified adviser.