The granny annexe, explained for families

What makes it an annexe

Three things set an annexe apart from a garden room or a home office. Someone sleeps there. It has its own kitchen and bathroom, so a person could live there day to day without crossing the garden. And it belongs to the main house: same plot, usually the same address, usually the same water, drains and electricity.

That last point matters more than it sounds. Planning permission for an annexe almost always comes with a condition that it stays part of the main home. It cannot be sold off or let to strangers. That condition is what keeps the application simple, and it is also why an annexe is cheaper to get approved than a new house in the garden.

Who it tends to suit

Most of the families who build one are planning for a parent in their seventies or eighties who wants to stay independent but no longer wants to live alone at the other end of the county. Others build for an adult son or daughter, or for a relative who needs a carer close by.

Before you look at any brochures, be honest about the next ten years, not the next two. Someone who walks well today may use a frame or a wheelchair later. Planning for that from the start costs far less than altering the building afterwards.

The kinds you can build

There are five common routes. They differ in price, in how long they take on site and in the planning route they need.

• Factory-built (modular): made in sections indoors and craned or wheeled into place. The shortest time on site of any route.
• Timber frame or SIPs, built on site: panels put up on prepared foundations. Copes better with awkward access than a finished module.
• Brick and block: built like a small bungalow. The slowest route, and the one that looks most like the house.
• Mobile-home style (twin-unit caravan): built to the legal size limits for a caravan, which can change the planning route. See our planning guide.
• Conversion: a garage or large outbuilding turned into living space. Cheaper on paper, but only if the walls, roof and floor are worth keeping.

How big it needs to be

Floor area drives the price more than anything else, so settle it early. These are the sizes families most often end up with, measured inside the walls.

A studio is the smallest that works well for one person. For a couple, or anyone who wants to close a door on the bed during the day, plan a separate bedroom. A second bedroom is usually for a carer, a visiting relative or a partner who sleeps badly.

What a granny annexe costs

Most finished annexes cost £40,000 to £100,000 including VAT. As a national guide that works out at £2,200 to £3,200 per m² of internal floor, with standard foundations included. The spread comes from the finish: a plain annexe with uPVC windows and a simple kitchen sits at the bottom, a wet room and aluminium doors in the middle, large glazing and joinery made to measure at the top.

That gives £44,000 to £64,000 for a 20 m² studio, £70,500 to £102,500 for a 32 m² one-bedroom annexe and £99,000 to £144,000 for 45 m² with two bedrooms, all including VAT. In London and the South East, allow 20 to 40% more. The cost calculator works it out for your own size and finish.

Then budget for the things a builder's headline price leaves out: the trench for water, drains and power from the house, planning and building control fees, and deeper foundations if the ground is soft or sloping.

Planning and building control

In England a self-contained annexe usually needs planning permission. The rules that let you put up a shed or summerhouse without asking cover buildings used for things incidental to the house, such as hobbies or storage. Sleeping, cooking and washing are not incidental, so they fall outside.

Most families apply with a householder application, which costs £548 in England. A twin-unit mobile home that meets the legal definition of a caravan can sometimes avoid an application, though a lawful development certificate is the safe way to confirm it. Our planning guide goes through both routes.

A fixed annexe also needs building regulations approval, which covers the structure, insulation, fire safety, drainage and access. In Scotland a fixed annexe needs planning permission and a building warrant before work starts.

Council tax and VAT

A self-contained annexe is normally given its own council tax band. In England, if a relative lives in it, the bill is halved; if that relative is 65 or over, or substantially and permanently disabled, it is not charged at all. Tell the council when the annexe is occupied rather than waiting for a bill.

VAT is charged at the standard 20% on a new annexe in the garden of an existing house, because it cannot be sold separately. Some work done to make the annexe usable for a disabled person can be zero-rated. Ask the builder to show it on the quote.

Designing for later life

These cost little at drawing stage and a great deal later.

• A level threshold at the front door, with no step and a path to it that does not slope steeply.
• A wet room, or a shower with a floor that drains level, in place of a tray to step into.
• Doors wide enough for a wheelchair, and space to turn one beside the bed and in the bathroom.
• Sockets and switches set at a height you can reach without bending low or stretching high.
• Blocking in the bathroom walls, so grab rails can be fixed later where they are needed.
• A window from the living room that looks towards the main house.

How long it takes

Planning is usually the longest wait. A council aims to decide a householder application within eight weeks, and design and drawings take a few weeks before that. On site, a factory-built annexe is usually in place and finished within weeks, while brick and block takes a few months. Most families should plan on well over half a year from first sketch to moving in.