Garden Room vs Extension London 2026: Cost, Planning & Value Compared
A garden room costs a fraction of an extension but adds less to your home's value. Here is the honest comparison for London homeowners in 2026: what each costs, what planning rules apply, and when each option genuinely makes more sense.
Quick Answer
Garden rooms (10–15m²) cost £15k–£40k and are usually permitted development. Extensions (12–20m²) cost £50k–£120k+ inc VAT all-in. Extensions add 10–25% to property value; garden rooms add 5–10% at best. Garden rooms take 2–6 weeks to build with almost no disruption. Extensions take 3–6 months and involve significant disruption.
£15k–£40k
Garden room (12m²)
£50k–£120k+
Extension (15m²)
4× faster
Build time difference
Check your specific property constraints
Free Property CheckThe Fundamental Difference
A garden room is a separate, self-contained outbuilding in your garden. It does not connect to the house. An extension physically attaches to the existing building, becoming part of the house itself.
This single distinction drives almost every other difference between the two: the planning rules, the cost, the build complexity, the value added, and the building regulations requirements. It also determines which option solves your actual problem.
Garden room
- Separate structure in the garden
- Own foundations (lightweight slab or screw piles)
- Independent electrics and heating
- No structural connection to the house
- Typically 8–20m²
Extension
- Physically attached to the house
- Full foundations (strip or trench fill)
- Integrates with existing services
- Structural opening into existing house
- Typically 12–36m²+
2026 Cost Comparison
The cost difference is significant. A garden room costs roughly a third to a fifth of an extension for a similar floor area. All figures below include VAT at 20%.
| Option | Floor area | Cost range | Cost per m² |
|---|---|---|---|
| Garden room (basic insulated) | 10–12m² | £15,000–£22,000 | £1,500–£2,000 |
| Garden room (high-spec with electrics) | 12–15m² | £25,000–£40,000 | £2,000–£2,800 |
| Single-storey extension (mid-range) | 12–15m² | £50,000–£80,000 | £3,600–£5,000 |
| Single-storey extension (high-spec) | 15–20m² | £80,000–£120,000+ | £4,500–£5,500 |
Why extensions cost so much more
Garden rooms avoid most of these costs. No structural opening, no party wall, lightweight foundations, and usually no planning application.
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Planning Permission: Garden Room vs Extension
Planning is one of the biggest practical differences. Garden rooms are almost always simpler from a planning perspective.
Garden room planning rules (Class E outbuildings)
Garden rooms fall under Class E of the General Permitted Development Order as outbuildings. You can build without any planning application provided:
- Maximum eaves height of 2.5m
- Maximum overall height of 4m (dual pitch) or 3m (any other roof type)
- Single storey only
- Not forward of the principal elevation (not in the front garden)
- All outbuildings and extensions together must not cover more than 50% of the garden
- Not used as self-contained living accommodation (no bedroom, kitchen, or bathroom for independent living)
Conservation areas: In a conservation area, outbuildings over 10m³ in volume need planning permission. A typical 3m × 4m garden room with a 2.5m eaves height is around 30m³, so most garden rooms in conservation areas do require a planning application.
Listed buildings: Any outbuilding in the grounds of a listed building requires listed building consent.
Extension planning rules
Extensions have a more complex planning landscape with three routes:
| Route | Max depth | Time | Cost |
|---|---|---|---|
| Permitted development | 3m (terraced/semi) or 4m (detached) | No wait | £0 |
| Prior approval | 6m (terraced/semi) or 8m (detached) | 6–8 weeks | £120 |
| Full planning | No fixed limit (merit-based) | 8–12 weeks | £528 |
For most London homeowners, a garden room is the easier planning route. No application, no waiting, no risk of refusal. Extensions under 3m depth are equally straightforward under PD, but larger extensions require prior approval or full planning.
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Check now — it's free →Value Added: Which Adds More to Your Home?
This is where extensions pull decisively ahead. An extension becomes part of the house. It shows up in floor area measurements. Estate agents include it in property descriptions. Buyers pay for it.
Extension: 10–25% property value uplift
A well-designed kitchen-diner extension consistently adds more value than it costs to build in most London boroughs. The space is fully integrated into the home, increases the measured floor area, and transforms the way the house is used every day.
Garden room: 5–10% value uplift (variable)
Garden rooms are a bonus for some buyers and irrelevant to others. They do not increase measured floor area. A high-quality insulated garden office may appeal to buyers who work from home, but a buyer with no use for it may see it as lost garden space. The value added is inconsistent.
If your primary goal is adding property value, an extension is almost always the better investment. The cost-to-value ratio for a garden room is weaker, though the lower initial outlay means you recoup your money more easily if you sell soon after.
Value comparison example
Value uplift varies significantly by borough, property type, and quality of finish. Inner London postcodes typically see stronger returns on extensions. Figures are indicative based on estate agent assessments.
When Each Option Makes More Sense
The right choice depends on what you need the space for. Some uses only work as extensions. Others are ideal for a garden room.
Garden room is better for
- Home office or studio
- Music room or podcast studio
- Home gym or yoga space
- Therapy or treatment room
- Art studio or workshop
- Teenager's study or retreat
Extension is better for
- Kitchen-diner (open plan)
- Extra bedroom
- Downstairs bathroom or WC
- Open-plan living space
- Utility room
- Any room needing plumbing or wet services
The key question is: does this space need to be connected to the house? If you need to walk to the kitchen, use the bathroom, or move between rooms during daily life, an extension is the answer. If the space serves a distinct purpose that benefits from separation (focused work, exercise, creative activity), a garden room works better.
Build Time and Disruption
For many London homeowners, disruption is the deciding factor. A garden room barely touches your daily life. An extension turns your home into a building site for months.
| Factor | Garden room | Extension |
|---|---|---|
| Total project time | 2–6 weeks on site | 3–6 months total |
| Pre-build prep | 1–2 weeks (base preparation) | 2–4 months (design, planning, party wall) |
| Disruption to daily life | Minimal (work is in the garden) | Significant (dust, noise, no kitchen for days) |
| Can you live in the house? | Yes, completely | Yes, but uncomfortable during structural work |
| Neighbour impact | Low | Moderate to high (scaffolding, skips, noise) |
Many bespoke garden room companies manufacture panels off-site and assemble on location in a few days. The foundation (typically a concrete slab or screw piles) takes a day or two. Electrics are run from the house consumer unit via an armoured cable buried in the garden. Total on-site disruption is measured in days, not months.
Building Regulations
All extensions require building regulations approval. Garden rooms are often exempt, but not always.
Garden rooms: usually exempt
A garden room is exempt from building regulations if it is under 15m² in floor area and does not contain sleeping accommodation. Between 15–30m², it is exempt only if it is at least 1m from any boundary or constructed of substantially non-combustible materials. Over 30m² or with sleeping accommodation, full building regulations apply.
Extensions: always require building regulations
Every extension needs building regulations approval covering structure, foundations, insulation, drainage, fire safety, ventilation, and electrics. A building control surveyor inspects at key stages and issues a completion certificate. Without this certificate, you may have problems when selling.
Even where garden rooms are exempt, the electrical installation should be carried out by a Part P registered electrician who can self-certify the work. This protects you for insurance and resale purposes.
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Heating, Insulation, and Year-Round Use
A garden room is only useful year-round if it is properly insulated and heated. Cheap, uninsulated garden buildings are unusable in winter and unbearable in summer. This is the most important specification decision for a garden room.
What makes a garden room year-round
A split-system air conditioner provides both heating and cooling and is the most popular choice for garden offices in London. Running costs are typically £150–£300 per year.
Extensions benefit from connecting to the existing central heating system. Underfloor heating is the standard choice for extension floors because it works well with the large glazed areas typical of kitchen-diner extensions.
The Hybrid Approach: Extend and Add a Garden Room
You do not have to choose one or the other. Many London homeowners extend the house for the kitchen-diner they need and add a garden room for a home office. This can be done in phases: the extension first (while you have builder access), the garden room later when budget allows.
Combined budget example
Remember the 50% garden coverage rule: extensions and outbuildings combined must not cover more than half the original garden area.
The hybrid approach works especially well when the extension solves the main living space problem and the garden room provides a dedicated workspace. Post-pandemic, this combination has become one of the most requested project briefs for London architects.
Summary: How to Decide
Choose a garden room if:
You need a separate workspace or studio, you want minimal disruption, your budget is under £40k, you do not need the space to connect to the house, and the space does not need plumbing. Garden rooms are faster, cheaper, and usually need no planning application.
Choose an extension if:
You need more kitchen, living, or bedroom space inside the house. You want to add meaningful property value. You need plumbing or wet services. The space must be integrated with how you live every day. Extensions cost more and take longer, but they transform the house itself.
Consider both if:
You need more living space and a separate workspace. Do the extension first for the biggest lifestyle improvement, then add a garden room when budget allows. Check that combined coverage stays within the 50% garden rule.
Frequently Asked Questions
How much does a garden room cost in London in 2026?
A basic insulated garden room (10–12m²) costs £15,000–£22,000. A high-spec garden room with full electrics, insulation, heating, and quality finishes (12–15m²) costs £25,000–£40,000. All figures include VAT. Prices vary depending on the manufacturer, specification, and site access. Budget an additional £1,200–£2,500 for air conditioning if you want heating and cooling.
Do I need planning permission for a garden room in London?
Usually no. Garden rooms fall under Class E permitted development as outbuildings. They must have a maximum eaves height of 2.5m, not be forward of the principal elevation, and all outbuildings plus extensions must not cover more than 50% of the garden. In conservation areas, outbuildings over 10m³ need planning permission. Listed buildings always need listed building consent.
Does a garden room add value to a house?
A quality insulated garden room adds roughly 5–10% to property value, though this varies significantly. Some buyers see a garden office as a genuine asset, particularly in London where working from home is common. Others are indifferent or would prefer the garden space. An extension typically adds 10–25% and is more consistently valued by buyers.
Do garden rooms need building regulations?
Garden rooms under 15m² that do not contain sleeping accommodation are exempt from building regulations. Between 15–30m², they must be at least 1m from any boundary or built from substantially non-combustible materials to be exempt. Over 30m² or with sleeping accommodation, full building regulations apply. The electrical installation should always be done by a Part P registered electrician regardless of exemption status.
Is a garden room cheaper than an extension?
Yes, significantly. A garden room costs roughly £1,500–£2,800 per m² compared to £3,600–£5,500 per m² for an extension in London. A 12m² garden room might cost £20k–£30k while a 12m² extension costs £50k–£72k all-in. The difference is driven by foundations, structural work, building regulations, and professional fees that extensions require.
Can I use a garden room as a bedroom?
You can physically use a garden room as a bedroom, but if it becomes sleeping accommodation it triggers building regulations requirements (fire safety, means of escape, insulation standards). It also cannot be classified as self-contained living accommodation under permitted development rules. If you want a habitable garden room for sleeping, expect to need building regulations approval and potentially planning permission.
How long does it take to build a garden room?
Most garden rooms take 2–6 weeks from start to finish on site. Many manufacturers pre-fabricate panels off-site and assemble on location in 3–5 days. Foundation preparation (concrete slab or screw piles) takes 1–2 days. Electrical connection typically takes a day. Compare this to 3–6 months total for an extension including design, planning, and construction.
Can I have a garden room and an extension?
Yes. The main constraint is the 50% garden coverage rule: all outbuildings and extensions combined must not cover more than half the original garden area. Many London homeowners extend for a kitchen-diner and add a garden room for a home office. Check your total garden coverage before committing to both.
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