Basement vs Loft Conversion vs Extension: Which Adds Most Space?

Home renovations · Basements · Lofts

Basement, loft or rear extension? How the three compare on cost per m², usable space, planning and build time, from an architect’s view and three London projects.

Last updated 13 October 2026 · AURA Architecture & Interiors, RIBA Chartered practice

The figures here are approximate and meant to give you an idea, not a quote. Every house and brief is different, so your costs and programme will depend on your project.

Basement vs loft conversion vs rear extension at a glance

Factor	Basement	Loft conversion	Rear extension
Approx. build cost per m² (AURA rate, ex VAT, build only)	£3,000 to £4,500 under the house, including underpinning	£1,500 to £2,500	£3,000 to £3,500; small extensions often more
How the m² is measured	Additional floor area	Floor with more than 1.5m head height	Additional floor area
Space and headroom	Can run under the whole house; borough rules limit depth and spread	Set by the roof shape; the new stair takes floor from below	Full-height space at garden level
Light	Lightwells, rooflights or glazed garden access	Dormers and rooflights	Large doors, glazing and rooflights
Garden lost	None under the house; boroughs cap how much garden a basement can take	None	The extension’s footprint
Usual planning route	Planning permission commonly needed for a new basement, plus borough basement policy	Often permitted development; never in a conservation area	Often permitted development up to 3m (4m detached), or 6m (8m) with prior approval
Party wall notices	Almost always, for excavation near neighbours	Often, for work on the party wall	Often, for foundations or new boundary walls
Disruption	The most: the dig and spoil come through or around the house	Mostly above you, until the stair breaks through	The back of the ground floor, usually the kitchen, is out of use
Real AURA programme	Rostrevor Road, Fulham (with rear and loft): 5 months design, 10 months build	Bronson Road, Wimbledon (with rear): 5 months design, 6 months build	Bronson Road, Wimbledon (same build)
Best for	Extra rooms when you can’t go up or out	Bedrooms and a bathroom	Kitchen, dining and living space opening to the garden

Basement vs loft conversion vs extension: which gives the most space for the money?

A loft conversion usually gives the most floor area for the money, because its rate per m² is the lowest of the three. A rear extension sits in the middle and gives the best space at garden level. A basement usually costs the most per m², but can add a whole floor without touching the garden or the roofline.

The rate is only half the comparison. What matters is the cost of each square metre you’ll actually use, and what that space is for.

A loft gives bedrooms and a bathroom. A rear extension gives a kitchen, dining and living space that opens to the garden. A basement suits rooms that don’t depend on a view, such as a snug, playroom, utility or wine room, and it can hold a bedroom if the escape rules are met.

The order can change on a real house. A loft under a low ridge may only give a small area above 1.5m, so the lowest rate buys fewer metres. A small rear extension costs more per m² than a large one. And a house in a conservation area has no permitted development for the loft, which adds a planning application to the cheapest option.

How do the three compare per square metre?

AURA’s approx. build rates, ex VAT, are £1,500 to £2,500 per m² for a loft, £3,000 to £3,500 per m² for a single-storey rear extension and £3,000 to £4,500 per m² for a basement under the house. Add VAT at 20%, professional fees and a healthy contingency on top. Your project sets where in each range you land.

Option	Approx. rate per m², ex VAT	Measured over	What moves it within the range
Loft conversion	£1,500 to £2,500	Floor with more than 1.5m head height	Roof shape, dormer size, steelwork, the new stair, bathroom spec
Single-storey rear extension	£3,000 to £3,500 (small extensions often more)	Additional floor area	Glazing, a small footprint, drains and ground conditions
Basement under the house	£3,000 to £4,500 (includes underpinning)	Additional floor area	Depth, groundwater, access, lightwells, spoil removal

A single-storey rear extension is approx. £3,000 to £3,500 per m², ex VAT. Small extensions often cost more per m², because foundations, steel and glazing are spread over fewer square metres. Our cost guide sets out what a house extension costs per m² and the rest of an extension budget, including 10 to 15% for professionals on top of the builder’s quote.

A basement under the house is approx. £3,000 to £4,500 per m², ex VAT, and that rate includes underpinning. It climbs with depth, groundwater, difficult access, lightwells and the volume of spoil to take away. Our guide to planning a basement conversion in London covers the surveys, waterproofing and daylight.

How much usable space does each one really give you?

A rear extension gives full-height space at garden level, square metre for square metre. A loft gives less than its footprint, because the sloping roof and the new stair both take floor. A basement can run under the whole house, but daylight has to be designed in through lightwells, rooflights or glazed doors to the garden.

In a loft, only the floor with more than 1.5m of head height counts, and the new stair lands on the floor below, usually taking space from a bedroom or the landing. Building regulations set a minimum clear headroom over that stair, with a slightly lower allowance for loft conversions, so the stair position is one of the first things we test. Our London loft conversion guide covers the different loft types and what each gives.

Below ground, the floor area can follow the house above. Light is the design question. At Trinity Road in Wandsworth, the fully glazed garden access is the main source of daylight for the lower-level living area. At Rostrevor Road in Fulham, a walk-on rooflight in the rear extension forms a terrace above, serving the playroom.

A rear extension gives the easiest space to use: level with the garden, with as much glass and rooflight as the design allows. The cost is the garden itself. Under permitted development, extensions and outbuildings can cover no more than half the land around the original house, and on a short London garden every metre counts.

Which needs planning permission, and does your borough matter?

Lofts and single-storey rear extensions often fall under permitted development, within set limits and outside conservation areas. A new basement commonly needs planning permission, and many London boroughs add their own basement policies on depth, garden and soil cover. Your borough matters most for a basement, and your street matters for all three.

A loft conversion is often permitted development within set volume limits, but never in a conservation area. A single-storey rear extension is often permitted development up to 3m (4m detached), or 6m (8m detached) with prior approval from the council outside conservation areas. Either way, a lawful development certificate confirms your scheme meets the conditions, and our guide sets out how planning permission and permitted development differ.

For basements, the Planning Portal says creating a new basement will commonly need planning permission. Converting an existing cellar is unlikely to need it, unless it creates a separate unit, changes the use, adds a lightwell or changes how the outside looks. Councils can then set their own limits on depth and size, and in SW London they do.

Borough	What the council’s basement policy says
Wandsworth	Multi-level basements aren’t appropriate. As good practice, keep at least half the back garden and at least 1m of soil above. Planning is usually required, and always for flats, listed buildings and lightwells
Hammersmith & Fulham	An Article 4 direction removes permitted development for basements across the borough, so planning permission is needed to create or extend one
Lambeth	One basement storey only. Beyond the house, only to the rear, no larger than the house’s footprint, and keeping at least 70% of the rear garden at its original level. A Stage 1 Basement Impact Assessment is required
Merton	No more than 50% of the front, rear or side garden, with 1.0m of permeable soil above any part under a garden

Wandsworth’s basement guidance is a clear example of how much detail a borough sets.

When a planning application is needed, a householder application costs £548 in England, proposed to rise to about £575 from 8 December 2026, subject to Parliament.

How long does each take, and can you stay at home?

On three AURA projects, design took 5 months each time. The build then ran 6 months for a rear extension plus dormer loft, 10 months for a basement, rear extension and loft, and 18 months for a whole-house project with a basement and loft. Lofts and rear extensions are easier to live through. A basement is the hardest.

Project	Works	Design	Build	Budget on the project page
Bronson Road, Wimbledon	Rear kitchen extension of 3 to 4m with two rooflights, and a dormer loft with a main bedroom suite and en-suite	5 months	6 months	£190,000 including VAT, FF&E and professional fees, for both works together
Rostrevor Road, Fulham	Small existing basement extended front and back, double-storey rear extension, loft with two dormers, whole-house refurbishment	5 months	10 months	Undisclosed (developer client)
Trinity Road, Wandsworth	Period house extended and refurbished over 8 split levels, basement extended, loft extended	5 months	18 months	Undisclosed (developer client)

Each build covers every part of that project together, so none of these figures times one option on its own. They do show the pattern: the more of the house the work touches, the longer it runs.

Before the build, our standard programme allows 5 weeks to reach a concept design and 2 weeks for the council to validate a planning application, then the council’s decision period. Party wall notices run alongside: one month’s notice for excavation near a neighbour, and two months for work to an existing party wall.

A loft conversion happens mostly above you, and the disruptive moment comes when the stair opens up from the floor below. A rear extension takes out the back of the ground floor, which is usually the kitchen, so plan a temporary kitchen elsewhere in the house. A basement is the most disruptive of the three, because the dig, the underpinning and the spoil all come through or around the house.

At Bronson Road, the clients lived in Singapore during the build, so AURA managed the project for them throughout. Planning approval for both the kitchen extension and the dormer loft came in August 2021.

One Fulham house, all three: what Rostrevor Road shows

Rostrevor Road in Fulham did all three: a small existing basement extended front and back, a double-storey rear extension, and a loft with two dormers. Design took 5 months and the build 10. It was a development project, for a client renovating to sell, so it’s evidence of what’s buildable and how long it takes. Its budget isn’t published.

The townhouse was very tired when the work began. Below ground, a very small existing basement was extended to the front, for a TV snug or bedroom, and to the back, for a new kitchen and dining area. A walk-on rooflight in the rear extension forms the terrace above, serving the playroom.

At the top, the loft conversion has two dormers and two rooflights, serving a double bedroom and an en-suite. The floors between were remodelled. The first floor holds the principal suite, with the bathroom moved to form a study, and the second floor has two double bedrooms and the main bathroom.

The 10 months covers the basement, the extension, the loft and the refurbishment of the whole house in one build. The project page shows the finished rooms at Rostrevor Road, level by level.

Trinity Road, Wandsworth

An early 1900s house in Wandsworth took the same idea further, across 8 split levels. Its basement was extended to form a large reception room, wine room, study, courtyard, utility and plant room, and the loft became two further double bedrooms and a family bathroom. Design took 5 months and the build 18. The award-winning Trinity Road development was built for a developer client.

When would we recommend each?

We’d recommend a loft when you need bedrooms and the roof has the height, and a rear extension when the brief is a kitchen and living space at garden level and the garden can spare the depth. A basement makes sense when you can’t go up or out, the street supports the spend, and you can accept the longest build.

A loft conversion. You need one or two bedrooms and a bathroom, the roof is tall enough, and the budget is the main limit. It leaves the garden and the ground floor as they are. In a conservation area it’ll need planning permission, and it’s still often the best value per m².

A rear extension. The kitchen is cramped or cut off from the garden, and the garden is deep enough to give up a few metres. It’s the option that reworks the floor you live on most and opens it to the outside. On a short garden, check what’s left before you fall for the plan.

A basement. There’s no room to go up, because the loft is done or the roof can’t change, and no room to go out, because the garden is short or the rules protect it. The house is on a street where values support the spend. You’re ready for the most technical build of the three, with more consultants, more reports and a longer programme.

Whether the spend comes back when you sell depends on the street, the house and what similar houses nearby already have. For many families the real comparison is with the cost of moving, which our guide to moving house or extending looks at in detail.

Should you combine them, and in what order?

If you’ll want two or three of these in the next few years, doing them in one build often makes sense: one set of scaffolding, one party wall process and one period of disruption. Phasing works too, as long as the whole house is planned first, so the first stage doesn’t make the second harder.

All three AURA projects in this article combined works. Bronson Road paired a rear extension with a dormer loft in one 6-month build. Trinity Road extended both the basement and the loft, and Rostrevor Road did all three.

Party walls show what combining saves. AURA’s fees guidance puts a party wall notice at approx. £200 and an award, if a neighbour doesn’t consent, at approx. £900 to £2,000+. On a terrace with neighbours on both sides, one round of notices beats one for each phase.

If you phase, the order usually follows the disruption. A basement dig and its underpinning affect the whole house, so they sit best before new finishes go in above. A loft is largely separate from the floors below, apart from the stair, so it can come first or last.

Combining does raise the total budget and the time the house is a building site. It suits households that can fund the whole scope at once and live elsewhere if the basement needs it.

What can go wrong, and how to avoid it?

Each option has one or two predictable risks: water and party walls below ground, headroom in the loft, garden and daylight at the back, and budget creep on all three. Each one is cheaper to settle on paper than on site, so the time to deal with them is during design, before a builder prices the work.

What can go wrong	Where	How to avoid it
Groundwater	Basement	Commission a ground investigation early, and have the waterproofing designed as part of the scheme
Reports and assessment costs	Basement	Budget for them before planning. Lambeth requires a Stage 1 Basement Impact Assessment and may pass the cost of independent assessment to the applicant
Not enough head height	Loft	Measure the roof before design starts, and check where the stair lands and the headroom over it
Losing too much garden or daylight	Rear extension	Draw the garden that’s left, check the half-the-land rule, and look at the light reaching your neighbours’ windows
Party wall notices served late	All three	Serve them early, at least one month before excavation and two months before work to a party wall
A public sewer under the extension	Rear extension, basement	Check the sewer records at survey. Building within 3m of a public sewer usually needs a build-over agreement with Thames Water
Budget creep	All three	Fix the brief and the drawings before tender, and keep a healthy contingency

Basements trigger party wall notices more often than people expect. Under the Party Wall etc. Act 1996, you need to give notice to excavate within 3 metres of a neighbour’s building if you’ll go deeper than their foundations, or within 6 metres if the dig cuts a 45-degree line down from the bottom of their foundations. The neighbour has 14 days to respond in writing, as the government’s party wall explanatory booklet sets out.

For a loft or rear extension under permitted development, get a lawful development certificate before you build. It’s the council’s written confirmation that the work is lawful, and buyers’ solicitors will ask for it.

Frequently asked questions

Sources