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Garden Room Building Regulations: When You Need Approval, and When You Need Planning Too

Garden room building regulations turn on two numbers and one question. Under 15 square metres of floor area with nobody sleeping in it, a detached garden room is normally exempt. Between 15 and 30 square metres it is still normally exempt, as long as nobody sleeps in it and it is either at least a metre from every boundary or built substantially of non-combustible materials. Over 30 square metres, or with a bed in it, you need approval. Planning permission is a separate test with its own height and position limits, and the electrics are regulated whatever the size. This page sets out both sets of rules for England, with the points that catch London gardens out.

Building regulations and planning permission are two different tests

People use the two terms interchangeably, but they answer different questions. Planning permission is about whether the building is acceptable where it sits: its size, height, position and use. Building regulations are about how it is built: structure, fire safety, insulation, ventilation, drainage and electrics. A garden room can need one, both or neither. Most well-planned garden offices in London need neither planning permission nor full building regulations approval, but they almost always involve notifiable electrical work.

The building regulations thresholds

The Building Regulations 2010 exempt certain small detached buildings. The Planning Portal summarises the position for garden buildings like this:

• Under 15m² floor area, no sleeping accommodation: building regulations will not normally apply.
• 15m² to 30m², no sleeping accommodation: you will not normally need approval, provided the building is either at least one metre from any boundary, or constructed substantially of non-combustible materials.
• Over 30m²: the exemption does not apply, so you need building regulations approval through your council's building control or a registered building control approver.
• Any sleeping accommodation: the exemption does not apply at any size.

Two details matter in practice. First, the area is the internal floor area, so measure inside the walls, not the footprint of the base. Second, the one-metre rule is about fire spread to the neighbours. A timber-clad room tucked into the back corner of a terraced garden, within a metre of two fences, falls outside the 15m² to 30m² exemption unless it is built substantially of non-combustible materials. In narrow London plots that is usually the deciding factor, and the reason many suppliers offer a “boundary” version with non-combustible cladding.

Electrics: Part P applies whatever the size

A garden room exempt from the rest of the regulations still needs safe, compliant electrics. Approved Document P covers electrical installations in a dwelling and in its garden and outbuildings. Running a new circuit from the house to the garden room, which almost every garden office needs, is notifiable work. Use an electrician registered with a competent person scheme, such as NICEIC or NAPIT, who can self-certify the work and register it with building control, or notify building control yourself before the work starts. Keep the certificate: a buyer's solicitor will ask for it. Our guide to Part P electrical regulations explains what counts as notifiable.

Plan the cable route early. An armoured cable buried at a safe depth, or run on a protected route, is normal, and it is far cheaper to lay before the base and paths go down.

Plumbing, a toilet or a kitchenette

Adding a WC, sink or shower brings in drainage rules under Part H. New connections to the drains are building regulations work, and a pump may be needed if the room sits below the level of the house drains. In terraced streets, check whether the drain you want to connect to is a shared sewer, which may need the water company's agreement before you dig. A toilet also makes the room look more like separate accommodation to a planning officer, which is the next issue.

When a garden room becomes an annexe

The moment anyone sleeps in the building, the small-building exemption falls away and full building regulations apply: fire safety and escape, insulation, ventilation, and the rest. Planning changes too. Permitted development only covers buildings used for a purpose incidental to enjoying the house, such as a home office, gym, studio or playroom. A self-contained annexe with a bedroom, bathroom and kitchen needs planning permission, and a separate home that could be let needs much more. Be honest about the use from the start; a retrospective application after a neighbour complains is far harder.

Planning permission: the permitted development limits

Outbuildings are normally permitted development, which means no planning application, as long as they meet the limits in Class E. The Planning Portal lists them as:

• no outbuilding on land forward of the principal elevation (usually the front of the house);
• single storey, with a maximum eaves height of 2.5 metres;
• maximum overall height of 4 metres with a dual-pitched roof, or 3 metres with any other roof;
• maximum height of 2.5 metres if within 2 metres of a boundary, which in most London gardens is the binding limit;
• no verandas, balconies or raised platforms over 0.3 metres high;
• no more than half the land around the original house covered by extensions and other buildings.

The extra restrictions that matter in London:

• Listed buildings: any outbuilding within the curtilage of a listed building needs planning permission. See our guide to listed building alterations.
• Conservation areas: an outbuilding at the side of the house needs permission. Many London boroughs also use Article 4 directions to remove some permitted development rights, so check with the council. Our conservation area renovation guide covers how to find out.
• Flats and maisonettes: permitted development for outbuildings applies to houses, not flats, so a garden room for a ground-floor flat needs a planning application. If you are a leaseholder, you will also need the freeholder's consent; see licence to alter.
• Trees: roots of a protected tree, or any tree in a conservation area, limit where a base can go.

If you want certainty, apply to the council for a Lawful Development Certificate. It confirms the building is permitted development, and it is useful when you sell.

Foundations, insulation and comfort

Exempt does not mean anything goes. A room you plan to sit in all winter needs a proper base (concrete, ground screws or piles, depending on soil and trees), a damp-proof layer, and insulation in the floor, walls and roof. Building regulations energy standards do not apply to an exempt building, but a cold, damp office is a poor use of money. Ask a supplier for the U-values of each element and the ventilation strategy; condensation is the most common complaint in the first winter.

A wood burner in a garden room raises separate fire and flue questions, especially close to a boundary. Talk to building control and use a HETAS-registered installer before you buy the stove. Our guide on hearth regulations covers the clearances.

Designing the inside

Once the regulations are settled, the design questions are about light, storage and how the room links to the house. We cover layout, glazing and finishes in how to design a garden room.

Sources: Planning Portal, is building regulations approval needed for an outbuilding?; Planning Portal, outbuildings: planning permission; GOV.UK, Approved Document P; MHCLG, permitted development rights for householders: technical guidance.