Extend or Just Renovate: One House, Two Prices
We priced the same Battersea terrace twice — with the rear extension and without. The gap was £38,532 and four weeks, and 68% of it sat in two trades.
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From our V&A Museum project — South Kensington, SW7
We priced the same Battersea terrace twice against the same architect's drawings — once with the single-storey rear extension, once without. The difference was £38,532 of a £169,780 net quotation, or 22.7%, and four weeks off an eighteen-week programme. The useful part is not the headline. It is that 68% of that saving sat in just two of thirteen sections, and two sections did not move at all.
Most people asking whether an extension is worth it are handed a rate per square metre. That answers a different question. What a homeowner with drawings in hand actually wants is: on my house, with my scope, what happens to the number if the extension comes out — and what happens to everything else?
This is that comparison, from a real tender. The client, the address and the consultants stay out of it; the area and the money do not.
The two prices
| What | The requirement | Source |
|---|---|---|
| With the rear extension | Full house refurbishment plus a single-storey infill side and rear kitchen extension. Eighteen-week programme. | £169,780.00 net |
| Renovation only | The same refurbishment, same first floor, no ground-floor extension. Fourteen-week programme. | £131,248.10 net |
| The difference | 22.7% of the larger figure, and four weeks of programme. | £38,531.90 net |
Approved Documents are statutory guidance to the Building Regulations 2010 for England. Compliance is judged against the Requirements in Schedule 1; the Approved Document shows one way of meeting them. Your building control body has the final word on your specific building.
Both figures are net of VAT, which is charged in addition at 20%. Both were measured from the same tender drawings rather than estimated, which is what makes the comparison worth anything: the only variable between them is the extension.
Where the £38,532 actually sits
The thirteen sections are identical in both quotations, so the difference can be read line by line. Sorted by how much each one moved:
| What | The requirement | Source |
|---|---|---|
| Build-ups | £42,205 with the extension, £25,961 without. Floor build-ups, the warm flat roof, cavity and party walls, partitions. | −£16,244 · 42% of the difference |
| Structure | £21,880 with, £11,890 without. The steel frame, foundations, padstones, fire protection. | −£9,990 · 26% |
| Preliminaries | £18,420 with, £14,800 without. Scaffold, protection, permits, waste, insurances — four fewer weeks of them. | −£3,620 · 9% |
| Drainage and waste | £6,280 with, £3,856 without. The extension built over a manhole; without it, the manhole stays where it is. | −£2,424 · 6% |
| Demolition | £7,980 with, £6,380 without. Less to strip out, and no excavation arisings. | −£1,600 · 4% |
| External works | £6,830 with, £5,362 without. No new garden boundary walls to either neighbour. | −£1,468 · 4% |
| Doors and windows | £7,345 with, £6,220 without. Fewer openings to weatherproof, no skylight. | −£1,125 · 3% |
| Kitchen and bathroom | £9,780 with, £9,150 without. The ground-floor WC comes out; the two first-floor bathrooms do not. | −£630 · 2% |
| Decorating | £15,942 with, £15,314 without. Slightly less wall area to skim and paint. | −£628 · 2% |
| Water and heating | £11,170 with, £10,710 without. | −£460 · 1% |
| Electrical | £14,928 with, £14,585 without. 120+ points becomes 115+. | −£343 · 1% |
| Gas | £2,170 either way. Same boiler, same flue, same certificate. | NO CHANGE |
| Joinery | £4,850 either way. Carried as a prime cost sum against the same joinery package. | NO CHANGE |
Approved Documents are statutory guidance to the Building Regulations 2010 for England. Compliance is judged against the Requirements in Schedule 1; the Approved Document shows one way of meeting them. Your building control body has the final word on your specific building.
The number that changes how you read a quote£26,234 of the £38,532 — 68% — is in structure and build-ups alone. An extension buys a hole in the ground, a steel frame, a roof and new fabric. It does not change the rewire, the boiler, the bathrooms or the decorating, because those are pricing the house you already have.
That is why "cost per square metre of extension" misleads people holding a whole- house scheme. Most of a refurbishment budget is not sensitive to whether you extend. Two sections here were identical to the penny, and a third moved by £343 on nearly £15,000.
Not extending is not the same as doing nothing structural
This is the part that gets missed, and it is the reason the saving was 22.7% rather than the 35% the client had in mind.
Taking the extension out did not take the structure out. The renovation-only option still carried:
If the reason you wanted the extension was an open, level kitchen, you are buying steel either way. What changes is how much of it, and whether it comes with foundations and a roof.
The consequence nobody prices: the party wall position
The extension required pockets cut into the party walls of both neighbouring houses. The renovation-only option did not.
That removes a whole class of notifiable work under the Party Wall etc. Act 1996, and with it a set of surveyor appointments, awards and schedules of condition on two sides. It does not necessarily remove the Act — excavation near a neighbour's foundations or work to a party structure can still engage it — but a scheme that stops at your own rear wall gives your neighbours materially less to dissent to. What that is worth in fees is set out in what a party wall agreement costs; what it is worth in programme is often larger, because a dissent runs a clock you do not control.
The honest line in the second quotation
The structural engineer's package on that job was designed for the extension. The renovation-only option does not build it.
So the beam and column forming the new rear opening were priced on our own allowance — roughly a five-metre universal beam and a 152x152x30 UC column — and flagged in the quotation as needing section sizes, a connection design and a pad-foundation design from the engineer before any steel is ordered, with those two lines re-priced against that design either way.
That is worth saying out loud because the alternative is common and looks better. A quotation that silently reused the extension's steel design would have read as more certain and been less true, and the difference would have surfaced as a variation once the real design arrived. If you are comparing two builders on an alternative scheme, ask which of them has told you what is still provisional.
What was separated out rather than deleted
Three items came out of the extension option but were priced individually, so they could be put back one at a time:
| What | The requirement | Source |
|---|---|---|
| Under-stair ground-floor WC | Sanitaryware, floor and wall tiling, extract, fire door, lighting, stud enclosure, skim and paint, step infill. | £3,050.00 |
| Garden boundary walls to both neighbours | 215mm facing-brick walls with footings, coping and making good, plus removing the existing rear fence. | £3,830.00 |
| Manhole relocation and new below-ground run | Relocate the manhole with gullies, waste, soil, rain and vent stacks, and form the new underground drainage run. | £1,620.00 |
Approved Documents are statutory guidance to the Building Regulations 2010 for England. Compliance is judged against the Requirements in Schedule 1; the Approved Document shows one way of meeting them. Your building control body has the final word on your specific building.
Pricing these as discrete add-backs rather than folding them into one reduced number is what lets a client actually choose. It also separates the two kinds of saving: money that goes away because the extension does, and money that is simply a deferred decision.
What this comparison does not tell you
For the general picture across London rather than one job, house extension cost in London 2026 sets out the ranges, and full house renovation cost prices the refurbishment side on its own. To put your own scheme through the same model we price real work with, the estimator itemises it by trade, net of VAT.
Our interest, declaredThe larger option is the larger contract, and we build extensions — so the version of this page that served us would push the extension. What actually serves you is asking your builder to price both, on the same drawings, on the same day, and to show you the sections side by side. If the difference is not concentrated the way it is above, ask why.
How to get this comparison for your own house
- 01Ask for both options against one set of drawings. Not two conversations weeks apart. The same measured quantities, the same rates, the same day — otherwise you are comparing two builders' moods rather than two schemes.
- 02Insist the sections line up. Thirteen sections in one quotation and eight in the other cannot be subtracted. If the structures differ, you cannot see where the money moved, which is the only thing worth seeing.
- 03Look for the sections that DON'T change. Gas, joinery, electrical and decorating should be close to identical. If they swing, the two options are not pricing the same house and something else has been changed quietly.
- 04Ask what structure survives the smaller option. Removing an extension rarely removes steel. Get the beam, the column, the foundation and the openings named and priced, so the smaller option is not sold as structure-free.
- 05Get the add-backs priced individually. A WC, a boundary wall, a drainage run. Separately priced, they are choices. Rolled into one reduced total, they are invisible and they come back as variations.
- 06Establish what the engineer actually designed. A structural package drawn for the extension does not size the alternative. Ask which lines are on the builder's allowance and when they get re-priced.
- 07Count the party wall consequences on both. Pockets into a neighbour's wall, or excavation below their foundations, are notifiable. A scheme that stops at your own rear wall may take two surveyor appointments off the table.
- 08Compare the programmes, not only the totals. Four weeks was the difference here. If you are renting elsewhere, that is a real number that belongs next to the build cost rather than under it.
The one line to take away
Ask for the same house priced both ways, and read the sections rather than the totals. On this job the extension changed two trades substantially, two not at all, and everything else by rounding — and that pattern told the client more about their decision than the 22.7% did.
2VP is the builder, not a marketplace: one contract, one named project lead, and no introducer fee stacked on the build cost. When a scheme has two versions, the same people price both, and you can see which lines moved.
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