The Extension Company designs and builds annexes for families who want a parent, grandparent or adult child close by, with their own front door. One team handles the design, the planning application, building regulations and the build, under one fixed, itemised price. North London projects are designed, managed and built by Pine Grove's own team.
This page covers what we build, what it costs, how we handle planning and how the process runs. If you are still working out what the rules allow, start with our guide to granny annexe rules and planning permission.
What we build
An annexe is a self-contained living space: its own bedroom, kitchen, bathroom and living area, or a studio layout that combines them. There are three ways to create one, and the right one depends on the plot.
- Garden annexe. A new detached building in the garden, with water, drainage, power and heating run from the house and finished to residential standard. The most flexible option, because the main house is not disturbed.
- Extension annexe. A self-contained suite attached to the house, usually a ground-floor extension with its own entrance, shower room and kitchenette, and a connecting door to the main home. It suits plots where the garden is too small for a separate building, or where you want everything under one roof.
- Garage or outbuilding conversion. An existing garage or garden building converted into an annexe, sometimes with a small extension. Often the most cost-effective route because the shell already exists, and usually the simplest for planning. Our garage conversions page covers the building work involved.
Every annexe is designed around the person who will live in it. For an elderly parent that usually means everything on one level, a level threshold, a walk-in shower, and doorways and turning space that work with a walking frame or wheelchair. For an adult child it is more often a compact studio or one-bedroom layout. We also look at where the building sits: far enough from the house for privacy, close enough for a sensible drainage run and an easy walk across the garden on a wet night.
We recommend a route after seeing your plot, the access to the back garden and how independent the space needs to be. For a sense of the garden buildings we take on, our Hadley Wood house extension project included a 55m² outbuilding in the garden alongside the main house works.
If nobody will sleep there, you do not need an annexe. A garden room used as an office, gym or studio is a simpler and cheaper project with an easier planning route.
How much does an annexe cost in London?
Annexe pricing is driven by the route, the size and the specification. These are typical 2026 ranges for a self-contained annexe, excluding VAT. The table covers the whole market, including factory-built units, so you can compare like with like.
| Type | What it is | Typical cost |
|---|---|---|
| Traditional garden annexe (brick & block) | Built on site to match the house | £70,000 – £120,000 |
| Extension annexe (attached, self-contained) | Ground-floor or two-storey with own entrance | £60,000 – £120,000 |
| Garage-to-annexe conversion | Convert existing garage, add kitchen & bathroom | £40,000 – £80,000 |
What moves a price within those ranges:
- Services. The kitchen, bathroom, drainage and heating are what lift an annexe above garden-room prices. A long drainage run from the back of the garden to the house drain costs more than a short one.
- Access. Materials and plant have to reach the back garden. A side passage wide enough for a small digger is cheaper to work with than carrying everything through the house.
- Specification. Kitchen, shower room, glazing and heating choices are the biggest swing once the shell is fixed.
- Building regulations. They always apply to an annexe, because it is habitable space, so proper insulation, ventilation, fire safety and drainage are part of every price.
Planning fees are separate and set nationally: from April 2026 a householder planning application is £548, and a lawful development certificate for proposed works is £274. Every fee is on our planning fees guide.
You get a fixed, itemised price after a free site visit, so the figure you sign is the figure you pay. For the neighbouring projects, see the garage conversion cost guide and the garden room cost guide.
How we handle planning
The first question is usually whether an annexe can be built without planning permission. For a new building that someone will live in, the answer is almost always no. The permitted development rules for garden buildings only cover uses incidental to the house, such as an office or a gym, and living accommodation is not one of them. So we plan for an application from the start, and do not sell you a shortcut that fails later.
What we do on every annexe:
- Check the address first. Planning history, conditions on the original permission, conservation area, Article 4 direction and listed status are checked before any design work.
- New garden annexe or extension annexe: a planning application. We prepare the drawings and a supporting statement that explains who will live there and how the annexe relates to the house, submit the householder application and deal with the planning officer. The council's target is eight weeks from validation. Approvals normally carry a condition that the annexe stays ancillary to the main house and is not let or sold separately.
- Garage or outbuilding conversion: usually a lawful development certificate. Converting an existing building into accommodation for a family member often needs no planning application. We apply for the certificate so you have the council's confirmation in writing.
- Building regulations on every annexe. Structure, fire safety, insulation, ventilation, drainage and electrics are approved and inspected, and you receive a completion certificate at the end.
The rules in full are in our granny annexe planning permission guide: incidental and ancillary use, what self-contained means, caravan-type mobile annexes, and what you can and cannot do with the annexe when you sell.
Council tax on an annexe
A self-contained annexe is normally given its own council tax band, but a family annexe rarely pays the full bill. In England the annexe gets a 50% discount where the person living in it is a relative of the person who pays council tax on the main house, or where the household uses it as part of the main home. It is fully exempt where it is the home of a dependent relative: someone aged 65 or over, severely mentally impaired, or substantially and permanently disabled.
Neither is automatic. You apply to your council once the annexe is occupied. We raise it at design stage so it is not a surprise later.
How it works
- Free site visit. We look at the garden, the access and the services, check the planning history of the address and talk through who the annexe is for.
- Design and fixed price. We design the annexe around the person who will live in it and give you a fixed, itemised price covering design, the applications and the build.
- Planning. We prepare and submit the planning application or lawful development certificate and handle the council's questions.
- Build. Built to the building regulations and an agreed programme, with one project lead from start to finish.
- Handover. A finished annexe with its building control completion certificate, plus the planning paperwork you will need for council tax and for any future sale.
Where we build annexes
North London first. North London annexe projects are designed, managed and built by Pine Grove's own team, from Cockfosters, Southgate and Winchmore Hill to Barnet, Enfield, Highgate, Muswell Hill, Crouch End and Palmers Green. Each area guide covers the local housing, the planning rules and the council's recent decisions, and the North London house extensions hub sets them out borough by borough.
We also take on annexe projects across the rest of London. A garden annexe needs a plot with room for it, so it suits the larger gardens of the outer suburbs. On a tighter plot, a garage conversion or an extension annexe is usually the better route. See every area we cover on our locations page.
Annexe guides
Each of these goes deeper on one part of the decision:
- Planning and the rules: granny annexe rules: planning permission, self-contained annexes and council tax · when an outbuilding stops being incidental, the test a lived-in annexe fails · planning permission for a garden room, for the garden-office side of the line
- Other routes to the same room: converting a garage into a bedroom · a garden room or an extension?