Garden Room Costs by Size and Specification
Prices below are supply-and-install averages for England (2026). London/South East costs run 15-25% higher. VAT at 20% is included.
| Size | Basic Summerhouse | Insulated Garden Room | Premium/Bespoke |
|---|---|---|---|
| 2.5 x 2.5 m (6 m²) | £3,000-£5,000 | £6,000-£9,000 | £10,000-£14,000 |
| 3 x 3 m (9 m²) | £4,500-£7,000 | £8,000-£12,000 | £14,000-£18,000 |
| 4 x 3 m (12 m²) | £6,000-£9,000 | £10,000-£16,000 | £18,000-£24,000 |
| 5 x 4 m (20 m²) | £9,000-£13,000 | £15,000-£22,000 | £24,000-£35,000 |
| 6 x 4 m (24 m²) | £11,000-£16,000 | £18,000-£28,000 | £28,000-£45,000 |
Prices are indicative. Groundworks, planning fees and specialist electrics are additional unless stated by your supplier.
What Affects the Cost of a Garden Room?
Several variables can push your budget up or down significantly:
- Size: the single biggest driver. Every extra square metre adds materials and labour.
- Insulation spec: year-round usability requires 70mm+ wall insulation, a warm roof and thermally broken frames, adding £2,000-£5,000 over a basic build.
- Cladding material: pressure-treated timber is cheapest; Siberian larch, composite board and render-effect finishes cost more but last longer with less maintenance.
- Glazing: uPVC doors/windows are standard; aluminium bi-fold or sliding doors add £1,500-£4,000.
- Electrics: a dedicated consumer unit with lighting, sockets and EV/heating supply typically costs £800-£2,000 for the electrical connection alone.
- Heating: electric panel heaters, a small air-source unit or underfloor heating add £500-£3,000.
- Base/foundations: a concrete slab costs £800-£2,500 depending on size and ground conditions; screw-pile or timber-frame bases are quicker but similar in cost.
- Location: London and South East labour rates add 15-25%; difficult access or sloping gardens raise groundworks costs.
- Planning permission: most garden rooms are permitted development, but listed buildings and conservation areas may require consent, adding £200-£500+ in fees.
Insulated Garden Room vs Basic Summerhouse
A basic summerhouse is an uninsulated timber structure — fine for occasional summer use but cold and damp from October to April. Prices start around £3,000 for a 6 m² flat-pack kit up to £10,000 for a larger bespoke version.
A fully insulated garden room is designed for year-round occupation. It typically features: 70-100mm mineral wool or rigid insulation in walls, floor and roof; a vapour barrier; double or triple glazing; and a proper electrical supply. These buildings genuinely serve as home offices, gyms or playrooms in all weathers and are the product most suppliers mean when they advertise 'garden rooms'.
The quality gap between the cheapest kit-build insulated rooms and mid-range bespoke suppliers is significant. Budget ranges should be stress-tested against build warranties (look for 10-year structural, 2-year electrical) and whether the floor is insulated from below — a common cost-cutting omission in cheaper builds.
Foundations and Base Costs
Every garden room needs a level, load-bearing base. Your main options are: a concrete slab (most durable, £800-£2,500 for up to 20 m²); a concrete pad-and-beam system; adjustable screw piles (fast, minimal excavation, £600-£2,000); and timber decking-style bases on adjustable feet (cheapest but less durable).
On sloping ground, expect groundworks costs to rise by £500-£2,000. If your soil is clay-heavy, a thicker slab or engineered solution may be required. Many garden room companies include a standard base in their price — confirm exactly what is and is not included before signing.
Planning Permission and Permitted Development
Most garden rooms are permitted development in England, meaning no planning application is needed, provided they meet these key rules: maximum eaves height of 2.5 m (ridge up to 4 m for a dual-pitch roof or 3 m otherwise); no more than 50% of the garden covered by outbuildings in total; not forward of the principal elevation; not used as a separate dwelling.
If your property is in a conservation area, an AONB, or is listed, permitted development rights are restricted and you should check with your local planning authority before ordering. Wales and Scotland have similar but slightly different PD rules. A planning application costs £258 in England (2026).
Watch Out for Hidden Costs
Always get a fully itemised quote. Common extras not included in headline prices: groundworks/base, electrical connection from the house, building regulations sign-off (required if the room exceeds 15 m² and is within 1 m of a boundary), delivery to difficult-access gardens, and decoration/flooring inside. These can add £2,000-£6,000 to the total.
Plan Your Budget Before You Buy
Our planner helps you set a realistic refurbishment reserve before you buy — so you can factor a garden room (and any other improvements) into your overall budget from day one, rather than being surprised after completion.