Loft conversion vs extension? For most London houses, a loft conversion is the cheaper way to add a bedroom and bathroom, adds more value for the money, keeps your garden and is less disruptive to live with. An extension is the better choice when what you really need is a bigger kitchen or living space on the ground floor, or when your loft simply isn’t tall enough to convert.
Many families end up doing both, often years apart. In this guide we compare a loft conversion vs extension on cost, value, space, planning and disruption, so you can decide which project to do first.
Loft conversion vs extension at a glance
| Loft conversion | Single-storey rear extension | |
|---|---|---|
| Typical London cost (2026) | £45,000–£75,000 for a rear dormer | £80,000–£150,000 for 20–30m², including fees and VAT |
| What you usually get | A bedroom and en-suite, or an office | A bigger kitchen, dining or living space |
| Added value | Up to 24% with a bedroom and bathroom | Around 5% for 10% more floor space |
| Garden | Untouched | Loses part of it |
| Planning permission | Often not needed (Permitted Development) | Often not needed for modest rear extensions |
| Time on site | Around 8–12 weeks | Often 3–4 months or more |
| Living at home during the build | Usually easy; most work is done from scaffolding | Harder; you often lose the kitchen for weeks |
| Biggest limit | Head height in the roof | Garden size and neighbours’ light |
The value figures are from Nationwide’s 2025 research; the extension costs are typical figures from published London cost guides. Your own numbers will depend on your house.
Cost and value: which gives the better return?
What each costs in London
A loft conversion reuses space you already have: the roof and walls are there, and there are no new foundations. That’s the main reason it costs less per square metre than an extension.
| Loft conversion type | Typical London cost (2026) |
|---|---|
| Rooflight (Velux) | £30,000–£45,000 |
| Rear dormer | £45,000–£75,000 |
| Hip-to-gable | £55,000–£80,000 |
| Mansard | £65,000–£110,000 |
An extension needs foundations, new walls, a new roof and usually large glazed doors, and it often comes with a new kitchen. Published London guides put build costs at roughly £2,800–£4,200 per m² for most projects, with a typical 20–30m² single-storey rear extension costing £80,000–£150,000 including fees and VAT, before the kitchen units. Our guide to the different types of loft conversion explains what you get for each loft price.
What each adds to your home’s value
Nationwide’s research found that bedrooms add more value than floor space alone:
- Adding a double bedroom and a bathroom to a three-bed, one-bathroom house can add up to 24%.
- Going from three to four bedrooms adds about 10%.
- Adding 10% more floor space without a new bedroom adds about 5%.
A loft conversion usually creates a bedroom and a bathroom, while a ground-floor extension usually creates living space. So in the loft conversion vs extension debate, the loft usually adds more value for each pound spent. Our guide does a loft conversion add value? has the full figures.
Planning permission, Building Regulations and party walls
Planning permission
Both projects can often be built under Permitted Development, without a full planning application, but the limits are different.
| Loft conversion | Single-storey rear extension | |
|---|---|---|
| Permitted Development limit | Up to 40m³ of extra roof space for a terraced house, 50m³ for a semi or detached house | Up to 3m deep for a terraced or semi-detached house, 4m for a detached house |
| Larger option | Bigger lofts (such as most mansards) need planning permission | Up to 6m (terraced or semi) or 8m (detached) through the prior approval process, where neighbours are notified |
| Height | No higher than the existing roof | Up to 4m high, lower near boundaries |
| Flats and conservation areas | Full planning permission usually needed | Full planning permission usually needed, and side or larger extensions aren’t allowed under Permitted Development in conservation areas |
Even when you don’t need planning permission, we recommend a Lawful Development Certificate as proof. Our loft conversion planning permission guide explains the process step by step.
Building Regulations
Both need Building Regulations approval. For a loft, the big issues are fire safety (a protected stairway, fire doors and interlinked smoke alarms), the new staircase and the structure; see our guide to loft conversion building regulations. For an extension, they’re foundations, drainage, structure and insulation.
Party walls
In a terraced or semi-detached house, both projects usually involve the Party Wall Act. A loft conversion normally means cutting into the party wall for steel beams (two months’ notice), while an extension often means digging foundations within 3m of the neighbour’s house (one month’s notice).
Disruption: living at home during the build
This is where a loft conversion has a clear advantage.
A loft conversion is built mostly from scaffolding outside the house. Materials go up the scaffold and through the roof, and the builders rarely need to come through your home until near the end, when the new staircase is fitted and the services are connected. Most families stay at home throughout, and the time on site is usually around 8–12 weeks.
An extension happens at the back of the house, usually where the kitchen is. The back wall is opened up, the garden becomes a building site, and you may be without a working kitchen for several weeks. Many families set up a temporary kitchen elsewhere, and some move out for part of the build. Extensions also tend to take longer on site, often three to four months or more, because of foundations, drainage and the fit-out.
Whichever you choose, the builder makes the biggest difference to how disruptive the job feels. Our guide on how to choose a builder explains what to ask, and our guide to house extension mistakes shows the common pitfalls with extensions.
Loft conversion vs extension: which is right for you?
The best way to decide is to start with what you need the space for.
| If you need… | Usually best | Why |
|---|---|---|
| An extra bedroom | Loft conversion | It’s naturally private and quiet, and adds the most value. |
| A bedroom with an en-suite | Loft conversion | Bedroom plus bathroom is where the biggest value increase comes from. |
| A home office | Loft conversion | It’s the quietest room in the house, away from family life. See our guide to the loft conversion home office. |
| A bigger kitchen or family room | Extension | Kitchen and living space belong on the ground floor, next to the garden. |
| Step-free space for an older relative | Extension | A ground-floor room avoids stairs. |
| To keep your garden | Loft conversion | It doesn’t use any outdoor space. |
| The most space for the money | Loft conversion | No foundations, and the roof structure is already there. |
When a loft conversion isn’t possible
A loft conversion vs extension choice isn’t always yours to make. A loft may not be suitable if the head height is too low (you need roughly 2.2m from the top of the ceiling joists to the ridge), if the roof structure makes it very costly, or if you live in a flat where the freeholder won’t agree (see our guide to loft conversions in flats). Our guide on whether your loft is suitable for conversion explains how to check.
Doing both
Many London families do both, usually the loft first, because it adds the most value and is the least disruptive, then the extension a few years later. If you plan to do both, tell your designer at the start, so the staircase, drainage and structure are planned for the second project.
FAQs
Is a loft conversion cheaper than an extension?
Usually, yes. In London a rear dormer loft conversion typically costs £45,000–£75,000, while a 20–30m² single-storey rear extension typically costs £80,000–£150,000 including fees and VAT. A loft reuses the existing roof and walls and needs no new foundations.
Which adds more value, a loft conversion or an extension?
In most cases a loft conversion, because it usually adds a bedroom and a bathroom. Nationwide found that adding a bedroom and bathroom can add up to 24% to a three-bed house, while 10% more floor space without a new bedroom adds about 5%.
Loft conversion vs extension: which is less disruptive?
A loft conversion. Most of the work is done from scaffolding outside, and families usually stay at home. An extension often means losing the kitchen and garden for several weeks.
Do I need planning permission for a loft conversion or an extension?
Often not, for either. Many lofts and modest rear extensions can be built under Permitted Development. Flats, conservation areas and larger projects usually need full planning permission.
How long does each take to build?
A loft conversion usually takes around 8–12 weeks on site. A single-storey rear extension often takes three to four months or more, depending on the size and the fit-out.
Can I do a loft conversion and an extension at the same time?
Yes, but it’s a bigger, more disruptive project, and you’ll almost certainly need to move out. Many families do the loft first and the extension later.
Not sure which is right for your home?
We’ll visit, look at your loft and your house, and tell you honestly whether a loft conversion makes sense, which type would work, and what it would cost. If your loft isn’t suitable, we’ll tell you that too.
