Garden room extension cost calculator
A garden room extension typically costs £1,800 to £3,000 per m² with VAT, so a 20 m² room comes to £36,000 to £60,000. Set your own width, depth and finish below.
3 m permitted development depth
Prices by size
Finished rooms with VAT, across the three types. National figures: London and the South East usually cost 20 to 40% more.
| Floor area | Garden room extension | Orangery | Conservatory |
|---|---|---|---|
| 12 m² | £21,500 to £36,000 | £19,000 to £36,000 | £13,000 to £21,500 |
| 16 m² | £29,000 to £48,000 | £25,500 to £48,000 | £17,500 to £29,000 |
| 20 m² | £36,000 to £60,000 | £32,000 to £60,000 | £22,000 to £36,000 |
| 25 m² | £45,000 to £75,000 | £40,000 to £75,000 | £27,500 to £45,000 |
| 30 m² | £54,000 to £90,000 | £48,000 to £90,000 | £33,000 to £54,000 |
| Per m² | £1,800 to £3,000 per m² | £1,600 to £3,000 per m² | £1,100 to £1,800 per m² |
Not sure which of the three you want? Orangery vs conservatory vs garden room extension sets out the differences.
What moves the price
Glazing. The largest single choice. Budget French doors and one roof light sit at the bottom of the range. Five metres of sliding doors, a glazed gable and two roof lights sit at the top.
The roof. A flat warm roof is the least expensive. A pitched, tiled roof with a vaulted ceiling costs more in structure and plastering but gives the room its height.
The opening into the house. A doorway that already exists is cheap to reuse. Taking out most of the back wall needs a steel beam, sometimes a steel post and new pad foundations, all sized by a structural engineer.
Ground and drains. Shrinkable clay, trees close by, made-up ground or a sewer under the footprint each add to the foundations.
Getting in. A terrace with no side passage means every barrow load goes through the house. Builders price that.
What the figure leaves out
| Fees and charges | Typical cost |
|---|---|
| Architect or architectural designer, planning drawings | £1,000 to £2,500 |
| Structural engineer | £600 to £1,200 |
| Building control | £700 to £1,100 |
| Party wall surveyor, each neighbour, one agreed surveyor | £700 to £1,500 |
| Householder planning application, England | £548 |
| Prior approval for a deeper rear extension, England | £249 |
| Lawful development certificate, England | £274 |
You will not need all of these. Many rooms need no planning fee at all, and a party wall surveyor only where you build on or close to a boundary.
Garden room extension cost questions
How much does a garden room extension cost?
Between £1,800 to £3,000 per m² including VAT, finished and decorated. A 20 m² room therefore costs £36,000 to £60,000. These are national guide figures; your own price depends on the ground, the access and above all the glazing.
What is included in these figures?
Foundations, floor, walls, roof, glazing, the opening into the house, electrics, heating, plastering and decoration. Design fees, council fees, a new kitchen, floor finishes above a basic allowance and landscaping are not.
Why is the range so wide?
Glass. A 5 m run of slim-framed sliding doors can cost several times what French doors with side panes do, and roof lights, lanterns and gable glazing all add. Ground conditions come second: clay, trees and drains near the footings mean deeper or engineered foundations.
Is a garden room extension cheaper than an orangery?
They overlap. An orangery runs £1,600 to £3,000 per m² and a garden room extension £1,800 to £3,000 per m². A plain garden room with a flat roof can undercut an orangery; one with a vaulted ceiling and a glazed gable will not.
Do I pay VAT on an extension?
Yes, at 20% on labour and materials when a VAT-registered builder does the work. The figures on this page already include it. Always check whether a quote is shown with or without VAT before comparing two.
How do I keep the cost down?
Keep the shape a simple rectangle, stay within permitted development so there is no planning wait, choose one large set of doors over several openings, and settle every decision before work starts. Changes made on site are the most expensive kind.
Where these prices and rules come from
Price ranges are typical national ranges for 2026, including VAT. London and the South East usually cost 20 to 40% more. A guide, not a quote. Planning fees are set by MHCLG from 1 April 2026. The party wall figure assumes one surveyor agreed by both sides; if your neighbour appoints their own, you pay for both. Further reading and the rules behind the calculator:
- Single-storey extension costs: a practical guide for homeowners (Federation of Master Builders)
- Conservatory costs: roof replacement, upgrades and extensions (Federation of Master Builders)
- Permitted development rights for householders: technical guidance (GOV.UK)
- Building regulations approval (GOV.UK)
- Lawful development certificates (GOV.UK)
- The Party Wall etc. Act 1996: explanatory booklet (GOV.UK)
Get a quote for the room you priced
Postcode first, then four quick questions about the room, then your contact details. It takes about a minute and costs nothing.
We are not a builder. We look for a firm that builds this kind of room near you and tell you its name. Nothing is passed on until you say so.
Got it, thank you.
- We look for a firm that builds glazed extensions around your postcode, then email or phone you with its name.
- If you are happy with that firm, we send your enquiry across. If not, it goes nowhere.
- The firm measures up at the house and gives you its price. You will not hear from us about anything else.