Building a Granny Annexe: Planning Permission and Costs

A guide to building a granny annexe in your garden, including planning permission rules, permitted development limits, council tax implications and typical costs.

A granny annexe - a self-contained living space in your garden for a family member - is an increasingly popular solution for multi-generational living. Whether you want to provide independence for elderly parents, accommodation for adult children, or a flexible space that can serve as a home office today and living space tomorrow, understanding the planning and practical considerations is essential.

What Is a Granny Annexe?

A granny annexe is a self-contained (or semi-contained) residential unit within the garden or grounds of an existing house. They typically include a bedroom, bathroom, kitchen or kitchenette, and living area. They range from simple single-room structures to fully equipped one or two-bedroom homes.

The key distinction is that a granny annexe is ancillary to the main dwelling - it is not a separate, independent house. This distinction affects both planning and council tax.

Planning Permission: Do You Need It?

Whether you need planning permission depends on the size, design, and use of the annexe.

Permitted Development Route

Garden buildings - including annexes - can often be built under permitted development rights for outbuildings. The key PD limits are:

• Single storey only - no upper floors or galleries
• Maximum height: 2.5m if within 2m of a boundary; otherwise 4m (dual pitched roof) or 3m (any other roof)
• Maximum coverage: outbuildings and extensions combined must not cover more than 50% of the curtilage
• Not forward of the principal elevation - must be to the rear or side
• No separate sleeping accommodation - technically, PD for outbuildings does not permit separate residential accommodation. However, using a garden building as ancillary living space (i.e. incidental to the main house) is generally accepted

The grey area is the distinction between an "outbuilding incidental to the enjoyment of the dwelling" (PD) and a "self-contained dwelling" (requires planning permission). In practice, if the annexe is used by a family member, shares services with the main house, and is not independently let, most councils accept it as ancillary use under PD.

When You Will Need Planning Permission

• The building exceeds PD size or height limits
• The building is designed as a fully independent dwelling (separate postal address, utility meters, etc.)
• Your property is in a conservation area and the building would be to the side of the house
• Your property is a listed building
• An Article 4 Direction applies
• Your PD rights have been exhausted (other outbuildings already exceed the 50% coverage limit)

If in doubt, apply for a Lawful Development Certificate or seek pre-application advice from your council.

Council Tax

This is one of the most common concerns. A granny annexe that is ancillary to the main house - used by a family member, sharing the garden, and not separately let - is generally not subject to a separate council tax bill. It is treated as part of the main dwelling.

However, if the annexe is used as an independent dwelling (separately let, for example), the Valuation Office Agency (VOA) may assign it a separate council tax band. An occupied annexe used by a family member may qualify for a 50% council tax discount under the annexe discount provisions introduced in 2014.

Contact your council's revenues department to clarify the position for your specific situation.

How Much Does a Granny Annexe Cost?

Costs vary enormously depending on the type of construction and specification:

Additional costs to budget for:

• Foundations: £5,000 – £15,000 depending on ground conditions
• Services: connecting water, electricity, drainage - £3,000 – £10,000
• Landscaping: paths, patio, garden reinstatement - £2,000 – £8,000
• Professional fees: architect, structural engineer, building control - £3,000 – £8,000

Design Options

Timber Frame / Log Cabin

The most affordable option. Modern timber frame kits are well-insulated and can be highly attractive. They are fast to build (often 4–8 weeks) and can include all the features of a permanent dwelling. However, check with your council and insurer that timber construction is acceptable for habitable accommodation.

Modular / Prefabricated

Factory-built modules delivered to site and craned into position. Offers consistent quality, fast installation (sometimes a single day for the main structure), and modern design. Increasingly popular for annexes due to speed and predictable costs.

Traditional Brick and Block

Built on-site using conventional materials. More expensive and slower, but offers the most flexibility in design and the most solid, permanent feel. Brick annexes integrate well with traditional houses and often add the most value.

Building Regulations

Any habitable building must comply with building regulations, regardless of whether planning permission is needed. Key requirements include:

• Structural stability and adequate foundations
• Fire safety - fire detection, escape routes, fire-resistant construction
• Thermal insulation - meeting current energy efficiency standards
• Ventilation - adequate ventilation to all rooms
• Drainage - connection to the existing foul and surface water drainage
• Electrical safety - Part P compliance
• Accessibility - Part M compliance (particularly relevant for elderly occupants)

Accessibility Design

If the annexe is for elderly family members, consider:

• Level access - no steps, with wheelchair-accessible doorways (minimum 850mm clear width)
• Wet room bathroom - level-access shower, grab rails, non-slip flooring
• Wide corridors and doorways - for future wheelchair or walking frame use
• Emergency call system - connected to the main house
• Good lighting and contrast - important for those with visual impairment

Tips for Success

1. Confirm planning status before ordering a building - get a Lawful Development Certificate or pre-app advice
2. Notify neighbours - even if PD, maintaining good relationships matters
3. Keep it ancillary - maintain a functional connection to the main house (shared garden access, no separate postal address) to avoid separate council tax
4. Think long-term - design for accessibility even if not immediately needed
5. Engage professionals - an architect can maximise space and ensure compliance

A granny annexe is an investment in family life - providing independence, proximity, and peace of mind. With the right planning and design, it can be a cost-effective alternative to care homes or renting a second property.