Annexes in London: Planning, Building Regulations and Design Guide

• Whether an annexe requires planning permission depends primarily on:
• •Whether it is physically attached to or detached from the main house
• •Whether it is to be used as a self-contained dwelling (separate household) or as ancillary accommodation (used in connection with the main house)
• •Its size, height, and relationship to the main house and boundary

**Attached annexe (extension with granny flat):** An extension to the main house that incorporates a self-contained annexe area (bedroom, kitchen, bathroom, separate entrance) is treated as an extension for planning purposes. If it falls within permitted development limits, planning permission is not required. However, where a planning application is submitted, the London Planning Authority will assess whether the 'annexe' is in fact a self-contained new dwelling — which has more significant planning implications.

• **Detached annexe in the garden:**
• Building a new detached structure in the garden for use as a granny flat, home office, or ancillary accommodation is covered by PD under Class E (permitted development within the curtilage of a dwellinghouse). Key PD limits for a garden building/annexe:
• •Maximum height: 4m (dual pitched roof) or 3m (other roof types)
• •Must not be forward of the principal elevation
• •Must not occupy more than 50% of the curtilage (including any other structures)
• •In Conservation Areas or designated land: PD for outbuildings is more restricted

However: **if the detached annexe is to be used as a separate self-contained dwelling (sleeping accommodation with its own kitchen and bathroom, occupied as a separate household)**, it is likely to require planning permission for the change of use regardless of its size. The planning authority assesses whether the structure constitutes a separate dwelling — and the Council Tax implications follow.

**The 'ancillary use' test:** Annexes that are used in connection with the main house (by family members, as overflow accommodation, or as a home office/gym) are treated as ancillary and do not constitute a separate dwelling. Where the annexe becomes occupied by a separate household — particularly for rent — it becomes a separate planning unit and Council Tax assessment.

**Converting a garage to an annexe:** Converting an integral or detached garage to an annexe (with sleeping accommodation, kitchen, and bathroom) triggers similar planning considerations. Building Regulations approval is always required. See the separate garage conversion guide.

All new annexe structures (whether attached or detached) require Building Regulations approval if they are to be used as habitable space.

• **For a new detached garden annexe:**
• •**Structure:** The structural design must comply with the relevant Approved Documents. Timber frame and SIP (Structural Insulated Panel) construction are common for garden annexes.
• •**Thermal insulation (Part L):** The annexe must meet current energy standards. This means a high level of insulation (walls U-value ≤0.18 W/m²K, floor ≤0.22 W/m²K, roof ≤0.15 W/m²K for a new build) and thermally efficient windows.
• •**Drainage:** The annexe must have foul drainage connected either to the main house drainage system or to a dedicated connection to the public sewer. Surface water drainage must be appropriately managed.
• •**Fire safety (Part B):** Means of escape from the sleeping accommodation, smoke alarms.
• •**Electrics (Part P):** All electrical installation must be notified or self-certified under Part P.
• •**Ventilation (Part F):** Adequate background and purge ventilation in all habitable rooms.

**For an attached annexe (extension):** The Building Regulations apply to the extension as for any extension — thermal, structural, fire safety, drainage. The new kitchen within the annexe will require a dedicated waste connection.

**Modular and prefabricated annexes:** A growing number of companies offer modular or prefabricated garden annexe structures that are delivered as largely complete modules and installed on a prepared base. These are typically building-regulation-compliant as supplied, with the foundation slab, connections, and any site-specific works carried out by a local contractor. They can reduce build time to 2–4 weeks on site vs. 12–20 weeks for a traditionally constructed annexe.

**Costs for an annexe in London:**

• •**Garden annexe (single room, kitchen/bathroom, 30–40m², timber frame or SIP, new build):** £60,000–£120,000 depending on specification, drainage requirements, and whether prefabricated or traditional build
• •**Conversion of existing detached garage to annexe (including all services, insulation, kitchen, bathroom):** £30,000–£55,000
• •**Conversion of integral garage to granny flat (bedroom, kitchen, shower room):** £25,000–£45,000
• •**Extension-based attached annexe (adding a new wing with bedroom, kitchen, bathroom, separate entrance):** £60,000–£150,000 depending on size and specification

**Council Tax implications:** If the annexe is used as a self-contained dwelling occupied as a separate household, the Valuation Office Agency (VOA) will assess it as a separate Council Tax banding. Where the annexe is occupied by a relative of the main house occupier and used in connection with the main house (not rented or let separately), there is a 50% Council Tax discount available on the annexe's separate assessment in many London boroughs — check with your borough.

**CIL (Community Infrastructure Levy):** In London boroughs where CIL is charged (most of them), a new self-contained dwelling (including an annexe assessed as a separate dwelling) will attract CIL. A garden annexe used ancillary to the main house may be exempt — but this should be confirmed with the borough before proceeding.

**Utilities:** The annexe typically shares water supply and drainage with the main house (via spur connections). Electricity can be supplied via a spur from the main house consumer unit (with a sub-meter if the occupant is paying separately) or a new metered connection from the grid. Gas supply to a separate building requires a new meter and supply from the gas network — for a garden annexe, an all-electric approach (heat pump or electric heating, induction hob) is often simpler and avoids the gas connection cost.