Cost comparison
A standard house extension in London costs £2,500–£4,500 per m² for a single-storey rear extension and £3,500–£6,000 per m² for a double-storey. A 20m² single-storey extension typically costs £55,000–£100,000 all-in including planning, structure, M&E, finishes and VAT. By contrast, a bespoke 20m² garden room costs £25,000–£45,000 — approximately 40–60% of an equivalent extension. However, the cost comparison per usable m² is not straightforward: an extension connects to the main house and can include kitchen or bathroom additions that a garden room cannot match without separate planning for services.
Planning and approvals
Most garden rooms are permitted development (no planning application required, subject to size and position limits). Most single-storey rear extensions are also permitted development under Class A, subject to depth limits (6m terraced/semi, 8m detached) and 4m height limits. Where planning is required — mansard lofts, large extensions in conservation areas, extensions requiring a neighbour consultation scheme — the garden room avoids this complexity entirely. Building regulations always apply to extensions. For garden rooms under 30m², building regs are often exempt (except Part P electrics). For homeowners wanting to avoid the planning and building control process, a garden room is significantly lower friction.
Which adds more value?
A well-specified house extension consistently adds more absolute value than a garden room — particularly where it adds a kitchen-diner, bedroom or bathroom. A 20m² kitchen extension that creates an open-plan kitchen-diner can add £50,000–£80,000 to a London semi-detached, often exceeding its build cost. A garden room adds £10,000–£25,000 typically — meaningful but rarely exceeding the build cost in capital terms. However, for properties that have already been extended or lack garden space for a further extension, a garden room can unlock the only feasible additional space — making the comparison moot. The best approach depends on your priorities: maximum floor space and value → extension; lowest cost, fastest delivery, least disruption → garden room.
Build time and disruption
A garden room (12–16 weeks) causes minimal disruption to the main house — groundworks are in the garden and no internal work is needed until final electrical connection. A house extension involves significant disruption: roof opening, structural works, loss of kitchen or living space during works, noise and dust throughout the build (typically 12–26 weeks for a single-storey). For families with young children or home-working occupants, the disruption difference is material. Many clients choose a garden room specifically for its low disruption profile, then plan a larger extension as a future phase.
