Cheap House Extensions UK (2026): Cheapest Options Ranked

What is the cheapest house extension?

For the lowest total bill, a lean-to conservatory or a garage conversion wins. For the lowest cost per m² of year-round living space, converting a garage or loft beats building anything new, because the walls, roof and foundations already exist. A new brick extension costs most per m², but is often the only way to get a bigger kitchen where you want it.

The ranking below answers both questions using our cost calculator method: Midlands build cost per m², plus design fees, a structural engineer, building control, a £274 lawful development certificate and 20% VAT. Conservatory and garden room figures are published installed prices.

Cheapest house extension options ranked by all-in cost

Rank	Option	Typical size	All-in cost (Midlands)	Approx. cost per m²	Best for
1	Lean-to conservatory (uPVC)	3m × 3m (9m²)	£8,000 – £13,000	£900 – £1,450	Lowest total spend; garden room or play space
2	Garage conversion, basic to standard	15m²	£10,000 – £20,000	£650 – £1,350	A bedroom, office or snug at the lowest cost per m²
3	Rooflight (Velux) loft conversion	25m²	£20,000 – £35,000	£800 – £1,400	A bedroom where the loft already has good headroom
4	Insulated garden room	3m × 3m to 4m × 3m	£15,000 – £30,000	£1,400 – £3,000	An office without disrupting the house
5	Small single storey rear extension, basic finish	3m × 4m (12m²)	£28,000 – £38,000	£2,350 – £3,150	Enlarging a kitchen or adding a ground-floor room
6	Timber frame single storey extension	20m²	£34,000 – £48,000 plus fees	£1,700 – £2,400 (build only)	Faster build, slightly lower build cost
7	Dormer loft conversion, basic finish	30m²	£41,500 – £62,000	£1,400 – £2,050	A bedroom and bathroom where headroom is poor
8	Single storey extension, basic finish	20m²	£45,500 – £60,000	£2,300 – £3,000	A kitchen-diner on a budget

Conversions give habitable space for roughly half the per-m² price of a new extension. The smallest new extensions are poor value per m², because drawings, the engineer, building control, scaffolding and opening up the back wall cost much the same whatever the floor area.

1. Garage conversion: the cheapest habitable room

If you have an attached or integral garage you do not park in, converting it is usually the cheapest proper room you can add. The structure, roof and slab already exist; the work is mostly a new front wall and window, insulation, electrics and heating. At a basic finish a 15m² single garage costs about £10,000–£15,000 all-in in the Midlands, or £13,000–£20,000 at a standard finish. Planning permission is not usually needed unless your permitted development rights have been removed, but building regulations approval always is. See our garage conversion cost guide for single and double garage prices.

2. Rooflight (Velux) loft conversion

A rooflight conversion leaves the roof shape alone and adds windows in the slope, so it is the cheapest type of loft conversion. Our calculator puts a 25m² rooflight loft at about £20,000–£35,000 all-in from basic to standard finish outside London, in line with the £22,500–£30,000 typical range in our loft conversion cost guide. The catch is headroom: if you cannot stand up comfortably in the middle of the loft now, you will probably need a dormer.

3. Lean-to conservatory or uPVC conservatory

For the lowest total spend, a lean-to conservatory is hard to beat. Published 2026 prices put a fitted 3m × 3m uPVC lean-to at about £8,000–£13,000 including VAT, and a larger Edwardian or Victorian style at about £11,000–£19,500. Most conservatories are permitted development, and one under 30m², at ground level and separated from the house by external-quality doors, may be exempt from building regulations, which saves on design and inspection fees.

The trade-off is comfort: a glazed or polycarbonate roof can be too hot in summer and cold in winter, and a solid tiled roof adds roughly £3,000–£6,000. Our lean-to conservatory cost guide and conservatory cost guide cover sizes and roof types.

4. Garden room

Not an extension, but for a home office often the cheapest route. A 3m × 3m insulated room averages about £19,700 installed and most cost £15,000–£30,000, usually without planning permission if under 2.5m high near a boundary. The base and power are often extras. See our garden room cost guide.

5. Small single storey rear extension under permitted development

The cheapest true extension is a single storey rear extension of up to 3m (attached house) or 4m (detached) built under permitted development. At a basic finish, a 3m × 4m extension costs about £28,000–£38,000 all-in in the Midlands and £35,500–£47,000 in London. Choosing a flat roof, uPVC or standard aluminium doors and a couple of rooflights keeps it at the basic end. Our single storey extension cost guide prices every common size.

6. Timber frame and modular extensions: are they cheaper?

Slightly, but less than the marketing suggests. BestBuilders puts timber frame extensions at about £1,700–£2,400 per m² against £1,900–£2,600 for blockwork, a saving of roughly £200 per m², or about £4,000 on a 20m² extension. MyJobQuote’s timber frame range is wider, at £1,600–£2,700 per m². The bigger gain is time: BestBuilders says a timber frame shell can be weathertight in 2–5 days once foundations are ready, and the build overall is 20–30% faster.

Fully modular extensions save even less. BestBuilders’ August 2026 analysis prices modular at £2,100–£3,300 per m² against £2,200–£3,200 for traditional; its 25m² example saves only about £1,500 (roughly 2%) once crane hire and a heavier slab are added, though time on site drops from about 14 weeks to 4. Planning and building regulations are the same either way, so choose modular for speed, not a big discount.

Cheap house extension ideas on a budget

These ideas cut the cost of any extension without making it worse to live in.

• Use the space you have first. Removing a wall between a kitchen and dining room to create a kitchen-diner can solve the problem without an extension. MyBuilder puts a full wall removal with a steel beam at about £2,500–£6,500 (August 2026), plus building control and making good.
• Build a simple rectangle. Corners, angled walls and roof valleys all add labour. A box with one roof plane is the cheapest shape to build.
• Choose a flat roof. On a 20m² extension a flat roof typically saves about £3,000–£7,000 compared with a pitched roof. Our flat roof extension guide compares EPDM and GRP coverings.
• Limit the glass. Aluminium bi-folds cost about £3,150–£7,200 fitted (MyJobQuote, September 2026) and wide slimline sliders £8,000–£20,000. One set of doors and two rooflights is the budget option.
• Keep the kitchen and bathroom where the pipes are. Moving soil stacks and drains adds cost fast. Get a CCTV drain survey (about £100–£400) before the design is fixed.
• Keep nibs of wall at each side of the opening so you need a single beam rather than a goalpost steel frame.
• Phase the fit-out. A budget kitchen costs about £8,000–£15,000 against £15,000–£30,000 for mid-range. You can upgrade worktops or flooring later, but you cannot cheaply change the structure.
• Do the finishing yourself if you are capable: decorating, flooring and landscaping are the easiest DIY savings. Leave structure, gas and electrics to qualified trades.

Use permitted development to avoid planning costs

Staying within permitted development (PD) is one of the simplest ways to keep a low cost house extension low. It avoids the £548 householder planning fee, the risk of refusal and around eight weeks of waiting. In England, a single storey rear extension is usually PD if it:

• extends no more than 3m beyond the original rear wall of an attached house, or 4m for a detached house;
• is no more than 4m high, with eaves no higher than 3m if within 2m of a boundary;
• together with other extensions and outbuildings, covers no more than half the land around the original house;
• uses materials similar in appearance to the existing house.

Prior approval lets you go to 6m (attached) or 8m (detached) for a £249 fee. Single storey side extensions can be PD if no wider than half the original house. Loft conversions are PD up to 40m³ of extra volume on a terrace and 50m³ on other houses. Flats and maisonettes have no PD rights, and conservation areas, National Parks and Article 4 directions limit them.

A lawful development certificate (£274) is cheap proof for buyers’ solicitors that the work was PD. PD only removes the need for planning permission: every extension still needs building regulations approval. Read our extension planning permission guide for the full rules.

Where cutting costs is a false economy

Some savings cost more within a few years, or when you sell.

• Skipping building regulations. Without a completion certificate, selling gets harder, and putting it right can mean opening up finished work.
• No structural engineer. An engineer costs £500–£2,500 for an extension. An undersized beam or poor foundation can cost far more to fix.
• Cheap flat roof coverings. Old-style felt fails sooner than modern EPDM or GRP, which typically last 20–50 and 25–40 years. A failed roof means ceilings and finishes too.
• Ignoring the drains. Building over a public sewer without your water company’s consent can block your completion certificate.
• Skipping party wall notices. Notice under the Party Wall etc. Act 1996 is a legal requirement for most semis and terraces. If a neighbour consents in writing, it costs nothing. See our party wall agreement guide.
• No contingency. Ground conditions, hidden drains and changes of mind are normal. Hold back 10% of the build cost.
• Choosing the lowest quote blindly. A very low quote often omits items that come back as extras. The Federation of Master Builders warns that if a quote seems too good to be true, it usually is.
• Uninsured or cash-only builders. A cash discount is no saving if the work has to be redone.

How to get cheaper extension quotes

Builders price uncertainty: the vaguer the brief, the bigger their allowance for risk.

1. Get full drawings and a written specification first. The FMB notes that detailed plans make for more accurate quotes, and the HomeOwners Alliance says drawings let you compare quotes like for like.
2. Send every builder the same pack and ask for an itemised price, so you can see where they differ.
3. Get at least three quotes. That is the FMB, HomeOwners Alliance and BestBuilders recommendation. Asking for more than four can put good builders off.
4. Be honest about your budget. The FMB advises telling builders up front; a good one will say whether the budget fits the work.
5. Allow time. The HomeOwners Alliance suggests two to four weeks for builders to price a tender, and good builders are often booked weeks ahead.
6. Ask local builders, whose finished jobs you can go and see.

Our guide to comparing house extension quotes explains what a good quotation should include, with a sample line-by-line breakdown and a checklist.

VAT and cheap extension quotes

Building work on an existing home is normally charged at 20% VAT. A sole trader whose turnover is below the £90,000 VAT registration threshold does not charge VAT, which is one reason some quotes look much cheaper. That is legitimate, but compare the same scope and check insurance and track record. A registered builder offering a “no VAT” price for cash is a warning sign. If you need to borrow, see how to finance an extension.

Does a cheap extension add value?

Usually, if it adds genuinely usable floor space. Nationwide research reported in October 2025 found a 10% increase in floor space adds around 5% to a typical home’s value. Conversions often give the best return because they add a bedroom or office for the lowest cost per m², though converting your only garage can put off buyers where parking is tight. Check sold prices for extended houses on your street first.

Cheap house extension FAQs

What is the cheapest type of house extension?

A lean-to conservatory has the lowest total cost, at about £8,000–£13,000 fitted for 3m × 3m. For a proper year-round room, a garage conversion is cheapest per m², at about £10,000–£20,000 all-in for a single garage outside London.

Can you build an extension for £20,000?

Not a new brick extension in most of the UK in 2026. Even a 12m² single storey extension at a basic finish costs about £28,000–£38,000 all-in outside London. £20,000 will usually cover a garage conversion, a conservatory or a basic rooflight loft conversion.

How much is a small cheap extension?

A 3m × 4m single storey extension costs about £28,000–£38,000 all-in at a basic finish in the Midlands, and around £35,500–£47,000 in London, including design, engineer, building control, a lawful development certificate and VAT.

Is a timber frame extension cheaper than brick?

A little. BestBuilders puts timber frame at about £200 per m² less than blockwork, roughly £4,000 on a 20m² extension, and the build is quicker. Modular extensions save even less once crane hire is included.

Is a conservatory cheaper than an extension?

Yes, by a wide margin. A fitted uPVC conservatory typically costs £9,000–£25,000, while a small single storey extension starts at about £28,000. But a conservatory is less comfortable year-round, so compare the cost per usable month, not just the price.

How do I find cheap house extension builders near me?

Ask local builders to price the same drawings and specification, check their insurance and recent work, and compare itemised quotes. A low price only counts as cheap if it includes everything the others do.

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