Garden annexes
A garden annexe stands apart from the house, usually at the far end of a lawn or beside the drive. Being detached gives privacy on both sides. It also brings two jobs an attached annexe avoids: getting the building to the plot, and getting water, drains and power out to it.
One-bedroom garden annexe, about 35 m²
£70,500 to £102,500
Including VAT, standard foundations included. A guide, not a quote.
Garden room or garden annexe?
The line between the two is drawn by use, not by looks. A garden room is somewhere you go during the day: an office, a gym, a studio. Once someone sleeps there and it has its own kitchen and bathroom, it is an annexe, and the planning and building control rules change with it.
If you are thinking of buying a garden room and adding a shower later, stop and price it as an annexe from the start. The insulation, glazing, fire safety and drainage an annexe needs are hard to retrofit, and a council that finds someone living in a building approved as an office can take enforcement action.
Build types compared
Each type below can make a good annexe. What usually decides it is the access to the back garden and how quickly you need it.
| Build type | Time on site | Best when |
|---|---|---|
| Factory-built modules | The shortest | A crane or wide vehicle can reach the plot, and you want the shortest disruption |
| Timber frame or SIPs, built on site | Longer | Access is only a side passage or a gate, so parts must be carried in |
| Brick and block | The longest | The council wants it to match the house, or you want the heaviest, quietest walls |
| Twin-unit mobile home | Days, once the base is ready | The caravan planning route suits you and the size limits are enough |
The order holds for a typical single-storey annexe; the actual weeks depend on the weather, the ground and the firm's programme.
Getting it into the garden
Measure the narrowest point between the road and the plot before anything else: a side gate, the gap between the house and the fence, a low branch. A finished module needs either a clear route wide enough for its lorry or a crane that can lift it over the house. Crane lifts are routine, but they add cost and need a road closure or a council licence on some streets.
Panels and blocks can be carried down a passage less than a metre wide. That is the main reason families with terraced or semi-detached houses often end up with an annexe built on site.
Foundations
The ground under the lawn is often the biggest unknown in the price. A firm should look at it, and on some sites dig a trial hole, before giving a final figure.
| Base | Used for | Effect on the price |
|---|---|---|
| Ground screws | Light timber annexes on firm ground; little digging | Usually inside the guide price |
| Strip footings | Firm, level ground; the usual base for brick and block | Inside the guide price |
| Concrete raft | Soft or variable ground, or close to trees | An extra, priced after a look at the ground |
| Piles | Clay near large trees, made ground, or a steep slope | Designed by an engineer; can add several thousand pounds |
Water, drains and power
The services usually come from the main house through a trench across the garden. The cost rises with the length of the run and with what lies in its way: a patio to lift, a tree to avoid, a drive to cross. Waste water is the hard part. If the annexe sits lower than the house drains, it needs a pump.
Ask each firm to show the service connections as a separate line on its price, with the length of trench they have allowed for. It is the line most often missing from a headline figure.
What a garden annexe costs
As a guide, £2,200 to £3,200 per m² of internal floor including VAT, for a finished annexe on a standard base. A 20 m² studio comes to £44,000 to £64,000; a 32 m² one-bedroom annexe £70,500 to £102,500. Then add the service trench and any extra foundation work. London and the South East usually cost 20 to 40% more.
Try your own size in the cost calculator.
Where in the garden
The far end of the garden gives the most privacy and usually the cheapest planning case, but the longest service trench and the longest walk on a dark winter evening. Nearer the house is easier to live with, easier to connect and easier to keep an eye on. Most families settle on somewhere in between, with a lit, level path.
Keep it at least a couple of metres from boundaries where you can, both for neighbours and for maintenance. Councils look harder at a building that overlooks next door's garden or towers over a fence.
Where our information comes from
Build prices are our own national guide ranges for 2026 for a finished annexe, drawn from published UK cost guides and shown including VAT. London and the South East usually cost 20 to 40% more. They are a guide, not a quote. Rules on planning, tax and building control come from the official sources below and are summarised for England unless we say otherwise.
- The Town and Country Planning (General Permitted Development) (England) Order 2015, Class E
- GOV.UK: permitted development rights for householders, technical guidance
- GOV.UK: building regulations approval
- Approved Document M: access to and use of buildings
- Caravan Sites Act 1968, section 13 (twin-unit caravans)
- Planning Portal: what a planning application costs
Garden annexe questions
How close to the boundary can a garden annexe be?
There is no single distance in planning law for an annexe that needs permission; the council judges the effect on neighbours. Building regulations limit windows and combustible cladding close to a boundary, which in practice often means keeping a fixed annexe a metre or more away.
Can I live in a garden annexe all year?
Yes, if it is built and approved as living accommodation. That means insulation, heating, ventilation and fire safety to building regulations, not a summerhouse with a heater.
Does a garden annexe need its own address?
Not usually. It shares the house's address, though some families ask the council for a name or letter such as "The Annexe" so post and deliveries find it.