The instinctive comparison, "extension cost" versus "price difference between homes", misses most of what actually determines which option is cheaper. This calculator adds in the transaction costs of moving and the added value of extending, so you're comparing genuine net costs rather than headline prices.
How this calculation works
The extending side treats your net cost as the total build cost plus an estimated disruption cost, minus whatever value you expect the extension to genuinely add, since that added value is effectively recovered when you eventually sell. The moving side calculates Stamp Duty Land Tax on the new, larger property using current standard residential rates, then adds the estate agent fee on your current sale, legal and survey costs for both the sale and purchase, and removal costs, none of which are recovered; they're pure transaction costs with no offsetting value.
Extensions very rarely add value pound for pound with what they cost, and the actual figure depends heavily on your local market's ceiling price for your property type and your specific extension's quality and integration. See our Extension Budgeting guide for a full discussion of how to estimate this realistically for your own property, rather than assuming your build cost and added value are the same number.
What tends to tip the decision either way
- How close you already are to your local ceiling price. If your area's most expensive comparable homes aren't much above your current value, an expensive extension may not be recoverable, tipping the balance toward moving instead.
- How much space you actually need. A modest single-storey extension is more likely to add close to its cost in value than a very large, high-specification project that pushes the property well outside the norm for the street.
- Your tolerance for disruption versus moving stress. Both options carry a real, non-financial cost, months of building work versus the process of selling, buying and physically moving, and this is worth weighing alongside the numbers above, not instead of them.
Since "value added" is the most uncertain number in this calculator, it's worth running the comparison once with an optimistic estimate and once with a more conservative one. If moving still wins even under a generous extension value assumption, that's a genuinely useful signal; if extending only wins under an optimistic assumption, treat the result with real caution.
Frequently asked questions
Does this tool include the additional dwellings stamp duty surcharge?
No, this assumes you sell your current home before or as part of buying the new one, so the standard residential rates apply rather than the second-home surcharge. If you plan to keep your current property, the surcharge would apply and the moving option would cost meaningfully more than shown here.
Why include a "disruption cost" for extending?
Living through a building project has real costs, alternative accommodation for part of the work, takeaways when the kitchen is out of action, and general inconvenience, that are easy to overlook when comparing a build quote against a moving cost estimate. Including even a rough estimate makes the comparison more honest.
What if I'm not sure how much value my extension will add?
Research comparable extended properties that have actually sold in your area, rather than relying on a generic rule of thumb, and see our Extension Budgeting and Loft Conversion ROI guides for a fuller discussion of how value added varies by project type and local market.
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About the author
✓ Editorially reviewed — all Poqet guides are checked for factual accuracy before publication and updated when UK rates or legislation change. Editorial Policy