Granny Annexes & Garden Annexes in North London
Last updated: June 2026 · Figures verified against live North London quotes and current council planning records.
Quick answer: A self-contained annexe with its own kitchen and bathroom costs £40,000–£120,000 in 2026 — modular at the lower end, traditional build higher. Unlike a garden office, an annexe used for independent living almost always needs full planning permission, not permitted development.
The Extension Company designs and builds annexes across North London — self-contained living spaces for elderly parents, returning adult children, or a long-term guest suite that pays for itself. Demand has surged as multi-generational living becomes the norm, and North London's mix of generous gardens and high house prices makes the area well suited to it. We are based in Cockfosters, work on fixed-price terms, and build through our partner Pinegrove. This page explains what an annexe actually is, the different ways to create one, what it costs, and — honestly — the planning rules that catch most people out.
What is an annexe?
An annexe is a self-contained living space with everything a small home needs: its own kitchen, its own bathroom, a living area and a bedroom (or a studio layout combining them). That self-contained part is the whole point — and also what separates an annexe, legally, from a garden office or a spare room.
A garden annexe is essentially a high-spec garden room with full services run to it: water, drainage, power and heating, finished to residential standard so someone can live there independently. It is the kitchen and bathroom, and the fact that someone sleeps and lives there, that change everything about the planning position compared with a home office you only work in.
People build annexes for a clear reason: to keep family close but independent. The most common drivers in North London are elderly parents who want their own front door but help nearby, and adult children who cannot yet afford to move out. An annexe gives both generations privacy and dignity that a converted spare bedroom never can.
Three ways to create an annexe
There is no single "annexe" — it depends on what you are starting with:
- Garden annexe — a new self-contained building in the garden. The most flexible option and the most common, because it does not disturb the main house. Modular and prefab builds sit at the lower end of the cost range; traditional brick-and-block builds at the higher end.
- Extension annexe — a self-contained suite attached to the house, often a ground-floor or two-storey extension with its own entrance, kitchenette and bathroom. Good where you want the annexe connected and weatherproofed into the main home.
- Garage-to-annexe conversion — converting an existing garage (and sometimes extending it) into a self-contained unit. Often the most cost-effective route if you have a suitable garage, because the shell already exists.
We will recommend the right one after seeing your plot, your garden access and how independent you need the space to be.
How much does an annexe cost?
Annexe pricing is driven by size, spec and method (modular vs traditional). These are realistic 2026 ranges, excluding VAT:
| Type | What it is | Typical cost |
|---|---|---|
| Modular / prefab garden annexe | Factory-built, craned in, services connected | £40,000 – £70,000 |
| 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 |
A few things worth knowing before you budget:
- The £40,000–£120,000 range spans modular at the bottom to a full traditional build at the top — spec is the biggest swing.
- A garden annexe is like a high-spec garden room plus full services — the kitchen, bathroom, drainage and heating are what lift it above garden-office prices.
- Building regulations always apply to an annexe, because it is habitable space — budget for proper insulation, ventilation, fire safety and drainage.
Get an instant ballpark for your plot on the Extension Builder.
The planning truth — be honest with yourself here
This is where most annexe projects hit reality, so we are direct about it. A garden office you only work in is often permitted development. A garden annexe someone lives in usually is not — and the difference is the sleeping and living use.
A self-contained annexe used for independent living usually needs full planning permission. It is not permitted development like a garden office, because permitted development rights cover outbuildings that are "incidental" or "ancillary" to the main house — and a space with its own kitchen, bathroom and bedroom, lived in independently, fails that test. The council treats it, in effect, as creating separate living accommodation.
There is one niche exception: a genuinely "mobile" annexe under the Caravan Act — a unit that meets the legal definition of a caravan (built off-site, within size limits, capable of being moved) can sometimes avoid a full application. It is a specialist route with strict rules, not a loophole to rely on, and it is not right for most people.
The honest position: assume your annexe needs full planning permission, and budget the design and application time accordingly. We would rather set that expectation now than have you discover it after you have committed. We prepare and submit the application as part of the project.
A note on council tax
Because an annexe can be a separate dwelling, council tax may apply to it as its own band — though there are reliefs in some circumstances, such as where a dependent relative lives there or the annexe is used as part of the main home. The rules are specific and worth checking with the local authority early. It rarely changes the decision to build, but it should not be a surprise on a bill later. We will flag it during design so you can look into your own situation.
Why multi-generational living is driving demand
The reason annexes are booming is demographic, not architectural. Families are choosing to live together but separately: ageing parents who want to stay out of care and keep their independence, and adult children priced out of their own place. North London — with its larger period plots in areas like Southgate, Winchmore Hill, Highgate and Muswell Hill, and some of the highest house prices in the country — is one of the strongest markets for it. An annexe lets a family pool one expensive plot across two households, with everyone keeping their own front door.
How it works with The Extension Company
- Free site visit. We assess your garden, access and how independent the space needs to be, and recommend garden, extension or garage-to-annexe.
- Design. Architect designs the annexe to suit the occupant — single-level for elderly parents, studio or one-bed for an adult child — with a fixed-price proposal.
- Planning. We prepare and submit the full planning application, and advise on the council tax position.
- Build. Our partner Pinegrove builds to a fixed price and an agreed programme, to full building regulations.
- Handover. A finished, warrantied, self-contained living space ready to move into.
Where we work
We design and build annexes right across North London — Cockfosters, Southgate, Winchmore Hill, Barnet, Enfield, Highgate, Muswell Hill, Crouch End, Palmers Green and the surrounding areas. The generous gardens of the suburbs and the premium values of the period streets both lend themselves to it. See every area we cover on our locations page.
Frequently asked questions
How much does a granny annexe cost in North London? A self-contained annexe with its own kitchen and bathroom costs £40,000–£120,000 in 2026, excluding VAT. Modular and prefab garden annexes sit at the lower end; traditional brick-and-block builds and attached extension annexes at the higher end. A garage conversion can be the most cost-effective route if you have a suitable garage.
Do I need planning permission for a garden annexe? Almost always, yes. Unlike a garden office, a self-contained annexe used for independent living usually needs full planning permission — its sleeping and living use means it fails the "incidental" or "ancillary" test that covers permitted-development outbuildings. We prepare and submit the application for you.
What is the difference between a garden office and an annexe? A garden office is a workspace and is often permitted development. An annexe has its own kitchen, bathroom and bedroom and is lived in independently — which is what triggers full planning permission and full building regulations. The services and the living use are the whole difference.
Will I have to pay council tax on an annexe? Possibly. Because an annexe can be a separate dwelling, council tax may apply to it as its own band — though reliefs exist in some cases, such as where a dependent relative lives there. The rules are specific, so we flag it during design and recommend checking your situation with the local authority.
Can an annexe be used to house an elderly parent? Yes — it is one of the most common reasons people build one. A single-level annexe gives an elderly parent their own front door, kitchen and bathroom while keeping family close by, and certain council tax reliefs can apply where a dependent relative lives there.
Keeping family close but independent? Get an instant ballpark on the Extension Builder, then book a free site visit and we will design an annexe around the people who will live in it. Call The Extension Company on 020 3051 9430.