Key Takeaways
- No research says one beats the other. Nationwide's up to 24% covers "a loft conversion or extension" adding a double bedroom and a bathroom to a three-bed, one-bath house (Nationwide, 2025).
- Loft conversions cost £25,000 to £100,000+; single-storey extensions £45,000 to £90,000+ (Checkatrade, Architecture for London and Resi, 2026).
- A dormer loft is usually permitted development. A side return or a deep rear extension usually is not.
- Extra floor area on its own is worth about half its proportion: 10% more space adds around 5% to price (Nationwide, 2025). Bedrooms are what move the number.
- Your street's ceiling price decides the return, not the direction you build in.
- If you want both, settle the fire escape strategy before you open up the ground floor.
When a London home runs out of room, the choice is usually build up or build out. Most articles tell you a loft adds more value; the research does not actually say that. It says the same thing about both, and the real difference is which room your house is short of, what your plot allows, and where your street's ceiling price sits. Here is the honest comparison.
Which one adds the room your house is actually short of?
That is the whole decision, and it is worth answering before any figure. A loft conversion adds upstairs space, normally a double bedroom and an en-suite, and takes nothing from the garden. An extension adds ground-floor living space, most often a wider, brighter kitchen-diner, and takes it out of the garden. Nationwide's research gives the baseline: a 10% increase in floor space adds around 5% to the price of a typical house, so area on its own is worth roughly half its proportion. What lifts a property into a higher bracket is a countable extra bedroom, ideally with a bathroom. An extension that only makes an existing kitchen bigger is a lifestyle purchase, and so is a loft room too small to market as a double.
How do the two compare on cost, time and disruption?
A simple loft is the cheapest route to a bedroom, and a large mansard is not. The headline ranges overlap almost completely, so compare the rooms delivered, not the price tags.
| Loft conversion (rear dormer) | Single-storey rear extension | |
|---|---|---|
| Typical London cost | £55,000 to £120,000 for a dormer, £25,000 to £45,000 rooflight only | £45,000 to £90,000, more for a side return or wrap-around |
| Build rate | £1,800 to £2,500 per m² | £2,000 to £2,800 per m² standard, £4,300+ premium |
| Space gained | 15 to 30 m² of usable floor, less anything under about 1.5m of height | 15 to 25 m² typical, limited by garden depth |
| What it delivers | A double bedroom, usually an en-suite | Kitchen-diner, family room, sometimes a downstairs WC |
| Planning route | Usually permitted development, 40 m³ terraced and 50 m³ semi or detached | Permitted development up to 3m (terraced or semi) or 4m (detached), 6m and 8m by prior approval |
| Time on site | 6 to 10 weeks | 10 to 16 weeks |
| Time end to end | 4 to 7 months | 5 to 9 months |
| Disruption | Most work is external under a temporary roof; you can usually stay put | Kitchen out of action for much of the build; garden becomes the site |
| Garden lost | None | Whatever you build on, permanently |
Two lines in that table settle more arguments than the money does. The garden is priced in outer London family markets, so an extension that swallows most of a small one can cost as much in appeal as it adds in area. And the loft is worked on from outside, while the extension takes your kitchen away for three months.
Which one adds more value?
Neither, as a rule, and anyone telling you otherwise is quoting a number nobody published. Nationwide's 2025 What Adds Value research puts the headline figure at up to 24% for "a loft conversion or extension" incorporating a large double bedroom and a bathroom on a three-bed, one-bath house. It deliberately does not separate the two routes, because the value comes from the rooms created, not from the direction of travel.
| What the work creates | Uplift, terraced house | On the London average of £545,000 |
|---|---|---|
| A double bedroom and a bathroom, on a three-bed one-bath house | up to 24% | around £131,000 |
| Two bedrooms to three | around 13% | around £71,000 |
| Three bedrooms to four | around 10% | around £54,500 |
| An additional bathroom on its own | around 6% | around £32,700 |
| 10% more floor area, no new bedroom | around 5% | around £27,250 |
We have found no credible dataset scoring a loft against an extension separately, and we are not going to invent one. Where you see a table claiming a loft adds 20% and an extension 11%, it was made up somewhere and copied since. What genuinely changes the answer is local: in a road of converted lofts the unconverted house is the discounted one, and in a road where every kitchen already opens onto the garden the dark galley is the one that sticks.
What is the ceiling price, and why does it settle this?
The ceiling price is the most any house of your type on your street sells for, however much has been spent on it. It is set by the road, the catchment and the walk to the station, not by your specification, and it caps both projects equally.
To find yours, use HM Land Registry price paid data and look at sold prices, not asking prices, over the last eighteen months, on your street and the two either side, for your house type. The best-finished property that actually completed is your cap. Within roughly 10% of it, neither route returns its cost. Then ask a local agent what a three-bed sells for on your road against a four-bed: that gap, not a national percentage, is your return. Note too that the London average fell 3.7% in the year to May 2026 while the UK rose 2.7%, so the base itself has been going the wrong way.
What rules each one out?
Plenty, and it is cheaper to find out now than after drawings. A loft is off the table or gets expensive below about 2.2m of clear height from ceiling joists to ridge, on a trussed rather than a traditional cut roof, where the only place the stair fits is an existing bedroom, or where you own a flat, which has no permitted development rights at all. Conservation area status and Article 4 directions remove roof rights across whole neighbourhoods.
An extension is curtailed by garden depth, by a public sewer needing a Thames Water build over agreement, by protected trees, and by rights of light where you build tight to a boundary. Both usually trigger the Party Wall etc. Act 1996 in a terrace, and both need Building Regulations sign-off whatever the planning route. If neither direction works, our basement conversion cost guide covers the remaining one.
Our recommendation
Extend if the ground floor is the problem. A dark galley kitchen with nowhere to eat is a daily irritation that also puts buyers off, and no amount of loft space fixes it. On a terrace with a usable side alley, a side return is normally the better version of that job than a deep rear extension, because it buys width and light rather than depth. See open-plan kitchen-diner ideas and our house extensions service.
Convert the loft if the house is short of bedrooms and the garden is small. This is the stronger financial case on most London stock, because the loft consumes nothing you already own and a dormer is usually permitted development. Make it a proper double with an en-suite, or the 24% case does not apply to you. Our guide to whether a loft conversion adds value works through the arithmetic.
If you want both, decide the fire strategy first. This is the mistake we get called in to fix. A loft turns a two-storey house into a three-storey house, which needs a protected escape route: an enclosed, fire-resisting stairway down to a final exit. An open-plan ground floor destroys it. Knock through without planning for the loft and you pay later for a fire-resisting lobby or a sprinkler system. Design them together even if you build them years apart. See our knock-through guide and structural alterations service.
If you are within 10% of the ceiling price, do neither for the money. Do it because you want the space, budget it as consumption, and spend less. Which renovations add the most value sets both routes against everything else, and flat owners should read adding value to a London flat instead. For a straight assessment, contact us or call 07472 424 226.