Annexes and Granny Flats: Planning Permission and Building Regulations

An annexe (or granny flat, granny annexe, dependent accommodation) is self-contained or semi-self-contained living space within the grounds of an existing house. It can be:

• Part of the main house — a converted garage, a ground-floor extension, or a loft conversion with its own entrance and facilities
• A detached building — a purpose-built structure in the garden with bedroom, bathroom, and kitchen/kitchenette facilities
• A converted outbuilding — a garage, barn, or other existing structure converted to living accommodation

The critical question is whether the accommodation is "ancillary" to the main dwelling (used by family members as part of the household) or a "separate dwelling" (an independent unit). This distinction determines the planning rules, council tax treatment, and building regulations requirements.

Annexes within the main house

If you are creating an annexe within the existing house — for example, converting a ground-floor room or adding an extension with an en-suite and kitchenette — this is usually treated as a standard extension or internal alteration. The normal permitted development rules apply.

However, if the annexe has its own front door and is completely self-contained (separate entrance, kitchen, bathroom, living space), the council may consider it a "material change of use" to create a separate dwelling. This requires planning permission regardless of PD rights.

Detached annexes in the garden

This is where most people get confused. There are two possible routes:

Route 1: Permitted development (Class E — outbuildings)
Garden buildings for purposes "incidental to the enjoyment of the dwelling house" are PD under Class E, subject to the normal outbuilding rules: maximum height 4m (pitched) or 3m (other), no more than 50% curtilage coverage, single storey, no balconies or raised platforms.

The key word is "incidental." A garden building used as a hobby room, home office, or gym is incidental. A self-contained dwelling with a kitchen, bathroom, and bedroom where someone lives independently is not incidental — it is a separate dwelling.

The planning position on garden annexes under PD is: A garden building with sleeping accommodation and basic facilities (a sink, a microwave) that is used by a family member as additional space ancillary to the main house may be PD. A fully self-contained unit with a separate kitchen, plumbed bathroom, independent heating, and separate utility meters is likely to be considered a separate dwelling, which is not PD.

Route 2: Planning permission
If your annexe is genuinely self-contained — separate entrance, full kitchen, bathroom, bedroom, its own heating system — you almost certainly need planning permission. Apply for householder planning permission (£548).

Councils will consider:

• Whether the annexe is genuinely ancillary to the main house (used by a family member, not let independently)
• The size and scale relative to the main house
• Impact on neighbours
• Whether it creates a precedent for infill development
• Whether it can be tied to the main house by condition (so it cannot later be sold or let separately)

Many councils will grant permission for an annexe with a condition that restricts its use to accommodation ancillary to the main dwelling.

Converted outbuildings

Converting an existing outbuilding (garage, barn, stable) to living accommodation is a material change of use and requires planning permission. The PD rights for outbuildings only cover use "incidental to the enjoyment of the dwelling house" — sleeping accommodation as a primary use is not incidental.

Every annexe needs building regulations approval, regardless of how it is classified for planning purposes. The requirements are extensive because you are creating habitable accommodation:

Structure (Part A): If the annexe is a new-build structure, it needs proper foundations, structural walls, and a roof designed to building regulations standards. If it is a conversion, the existing structure must be assessed and upgraded if necessary.

Fire safety (Part B): The annexe needs mains-powered smoke detection, fire-resistant construction where close to boundaries, and adequate means of escape (openable windows in bedrooms meeting minimum size requirements). If the annexe is attached to the main house, the fire separation between the two must achieve a minimum fire resistance.

Damp, weather, and ground moisture (Part C): Proper damp-proof course, damp-proof membrane, and weather protection. Ground floors need insulation and a DPC.

Thermal performance (Part L): Walls, floor, roof, windows, and doors must all meet current insulation standards. For a new-build annexe, this means U-values equivalent to a new-build dwelling.

Drainage (Part H): Foul drainage from the bathroom and kitchen must connect to the existing drainage system or to a new connection approved by the water company.

Ventilation (Part F): All habitable rooms need background and purge ventilation. Kitchens and bathrooms need mechanical extract ventilation.

Electrical safety (Part P): All new electrical work must comply. Use a competent person scheme electrician (NICEIC, NAPIT, ELECSA) for self-certification.

Water efficiency (Part G): The annexe must not use more than 125 litres of water per person per day (or 110 litres in some areas).

Access (Part M): If the annexe is at ground level, it should meet accessibility standards — level thresholds, adequate door widths, and accessible bathroom layout.

We recommend the Full Plans route for annexes because of the breadth of regulations that apply.

This catches many homeowners by surprise.

Ancillary accommodation that is part of the same household (a granny annexe used by a family member, with shared use of the garden and facilities) should not attract a separate council tax charge. It may qualify for a council tax exemption or a council tax annex discount (usually 50% off the annexe's band).

A separate dwelling — one that is self-contained and could be used independently — will be assessed as a separate property for council tax purposes and will be charged a separate council tax bill at the appropriate band.

The Valuation Office Agency (VOA) decides whether the annexe is a separate dwelling for council tax purposes. Factors they consider include:

• Whether it has its own entrance
• Whether it has its own kitchen and bathroom
• Whether it has separate utility meters
• Whether it is physically separated from the main house
• Whether it could be let or sold independently

The practical implication: If you want to avoid a separate council tax bill, design the annexe to maintain some dependency on the main house — a shared entrance, a connecting door, shared utility meters, or no full kitchen (a kitchenette rather than a full kitchen).

Garage conversion

Converting an attached or detached garage to living accommodation. Usually needs planning permission (change of use) and always needs building regulations. Cost: £15,000–£40,000 depending on size and specification. Read our garage conversion guide for more detail.

Garden room annexe

A new-build single-storey structure in the garden. Can be timber-frame, SIP panel, or modular construction. Most are purpose-designed with bedroom, bathroom, and kitchenette. Cost: £40,000–£100,000+ depending on size, specification, and whether fully self-contained. May need planning permission if self-contained.

Extension annexe

An extension to the main house with its own entrance and self-contained facilities. This follows the normal extension rules for PD but may need planning permission if it creates a separate dwelling. Cost: £30,000–£80,000+.

First-floor conversion

Converting existing first-floor space (above a garage, for example) into an annexe with an external staircase. Usually needs planning permission and building regulations. Cost: £25,000–£60,000.

Professional fees on top: architect or designer (£1,500–£4,000), structural engineer if needed (£400–£800), planning application (£548), building regulations (£500–£1,000), drainage connection (£1,000–£3,000).

• Assuming it is PD because it is in the garden. Garden buildings under PD must be "incidental to the enjoyment of the dwelling." Self-contained living accommodation is not incidental. Check before you build.
• Not considering council tax. A fully self-contained annexe may attract a separate council tax bill. Design accordingly if this matters to you.
• Ignoring building regulations. Even if the annexe is PD for planning purposes, building regulations still apply. A garden building converted to sleeping accommodation without building regs approval will cause problems when you sell.
• Letting the annexe independently. If planning permission was granted with a condition restricting use to ancillary accommodation, letting the annexe to a non-family member is a breach of planning control.
• Underestimating services costs. Running water, drainage, electricity, and heating to a detached garden annexe is expensive. Budget £5,000–£15,000 for services depending on the distance from the main house.