Garden Room, Conservatory, or Extension: Which Is Right for Your London Home?

When a London household needs more usable space and has a garden, three broad options are available: a detached garden room (a separate structure in the garden), a conservatory (a glazed lean-to attached to the house), or a proper single-storey extension (a masonry or timber-frame addition built to the same standard as the main house). Each delivers something different at a different cost and with different planning implications.

The Detached Garden Room

A detached garden room is a self-contained structure built in the garden, separate from the main house. It typically sits on a timber, concrete, or ground-screw foundation and is clad externally in timber, composite cladding, or similar materials. It is used as a home office, gym, hobby room, studio, or occasional guest accommodation.

Planning position: Garden rooms in London are generally Permitted Development (no planning permission required) provided they:

• Are single storey with a maximum eaves height of 2.5m and maximum overall height of 4m (3m if within 2m of a boundary)
• Do not cover more than 50% of the total garden area (including any existing outbuildings)
• Are not used as a dwelling (no sleeping accommodation as principal use)
• Are not in front of the main building elevation

In Conservation Areas, planning permission may be required — confirm with the local planning authority.

Building Regulations: A detached garden room of less than 15m² floor area (with no sleeping accommodation) is typically exempt from Building Regulations. A room over 15m² requires Building Regulations approval, particularly for structural adequacy, insulation, and electrical installation.

Cost: A good-quality insulated garden room in London, 3m × 4m (12m²), typically costs £12,000–£25,000 fully fitted. Larger or higher-specification rooms can reach £40,000+. This is a wide range because the quality variation in this market is extreme — cheap garden rooms are poorly insulated, use thin cladding, and deteriorate quickly.

Best for: Home working; creative or hobby space; accommodation for older children or guests that benefits from physical separation from the main house; situation where the budget does not stretch to a full extension.

Limitations: It is a separate building — you go outside to reach it. It cannot form part of the heated, connected living space of the main house without significant additional work. It cannot add to the kitchen or living accommodation in the way an extension can.

The Conservatory

A conservatory is a glazed lean-to structure attached to the rear or side of the main house. It is defined in Building Regulations terms as a structure where more than 75% of the roof is translucent or transparent and more than 50% of the external walls are glazed.

Planning position: A conservatory that meets the criteria for a single-storey rear extension (Permitted Development: maximum 4m depth for detached, 3m for attached houses in London; maximum 4m height; not projecting beyond principal elevation) is generally Permitted Development. Larger conservatories, or those in Conservation Areas, require planning permission.

Building Regulations: A conservatory is exempt from Building Regulations provided it:

• Has a floor area of no more than 30m²
• Is separated from the main house by an external-quality door (so it can be closed off thermally)
• Does not extend into the main house heating system in a way that makes the exemption inappropriate

Cost: A uPVC conservatory in London, 3m × 4m (12m²), typically costs £15,000–£30,000 supply and installed. Aluminium or timber conservatories cost more. The wide price range reflects both specification and quality — a conservatory from a national double-glazing company is a very different product from a bespoke timber or aluminium structure designed by an architect.

Best for: Additional informal living space in a property that has reasonable garden sun exposure; budget-constrained situation where a full extension is not affordable; situations where the exemption from Building Regulations is important.

Limitations: The thermal performance of a conservatory is poor compared to a properly insulated extension. It is too hot in summer, too cold in winter, and barely usable outside a narrow temperature range without additional HVAC. For a London property that gets limited direct sunlight (north-facing rear, overshadowed by neighbouring properties), a conservatory is a poor investment. Modern glazing has improved but not solved this fundamental limitation.

The Single-Storey Extension

A proper single-storey extension is built to the same standard as the main house — masonry walls, insulated roof and floor, connected to the main heating system. It creates a room that is comfortable to use year-round and indistinguishable in quality from the original house.

Planning position: Subject to the same Permitted Development rules as any single-storey extension (maximum 3m depth for attached houses under the standard PD rules, larger under the prior approval householder application process). Conservation Areas and other constraints may require planning permission.

Building Regulations: Full Building Regulations approval required. The extension must meet current standards for structure, insulation (Part L), drainage, ventilation, fire safety, and electrical installation.

Cost: A single-storey rear extension in London, 3m × 5m (15m²), typically costs £35,000–£65,000 for a standard specification with kitchen or living room use. The higher cost compared to a conservatory reflects the full insulated construction, concrete foundations, structural openings in the rear wall to connect to the main house, and properly integrated services.

Best for: Genuinely extending the usable living space of the property; kitchen extensions; additional reception space; any situation where year-round comfort is required; adding value to the property in a meaningful way.

Limitations: Higher cost than a garden room or conservatory; requires planning or prior approval process in some cases; more disruptive to build.

The Decision Framework

Choose a garden room when: the primary need is a workspace or hobby space that benefits from separation; the budget is constrained; or a detached structure is more practical than extending the main building.

Choose a conservatory when: additional informal space is wanted at a lower cost than an extension; the property has a south-facing rear with good sun exposure; and a year-round, highly insulated room is not the primary goal.

Choose a single-storey extension when: the objective is genuinely to extend the kitchen or living accommodation; year-round comfort matters; the investment needs to add substantive value to the property; or the quality and permanence of the addition is important.

RCB Design & Build

RCB builds single-storey extensions across London and can advise on the relative merits of extension vs. garden room vs. conservatory for your specific property and needs. Contact us to discuss your space requirements.