A garden room is usually cheaper and quicker, and it suits a home office, gym or studio. A house extension costs more per square metre, but it becomes part of your home. It can hold a kitchen-diner, a bedroom or a utility room, and buyers and surveyors treat it as proper living space. If you need the space to be part of the house, extend. If you need a separate room for one clear purpose, a garden room is often enough.
This guide compares the two on cost, planning, build time, resale value and daily use. It also covers where a garage conversion fits in, because many Birmingham and Solihull homeowners weigh up all three.
TL;DR
- Cost: published 2026 figures put a garden room at roughly 30% to 50% below a like-for-like extension, but utility connections and groundworks narrow the gap.
- Planning: many garden rooms fit permitted development, but anything used as a bedroom or self-contained living space is treated differently and needs a closer look.
- Value: an extension adds formal living space. A garden room adds buyer appeal, which is harder to put a number on.
- Use: choose a garden room for a separate, single-purpose room. Choose an extension for a kitchen-diner, extra bedroom or anything that must connect to the house.
- Garage: converting an integral garage is usually the cheapest way to gain a room, if you can live without the garage.
Table of Contents
- What each option actually is
- Cost: where the savings are real and where they shrink
- Planning permission and building regulations
- Build time and disruption
- Resale value and mortgages
- Which one fits your situation
- Garage conversion vs extension
- Our perspective
- Talk to us about your extension
- FAQ
- Sources
What each option actually is
A garden room is a separate, insulated outbuilding in your garden. Which? describes it as an outdoor building that sits apart from the house but works as extra space. Most are timber-framed or SIPs-built, sit on ground screws or a slab, and arrive with lighting, heating and power.
A house extension is built onto your home with foundations, masonry walls and a roof tied into the existing structure. Typical forms in the West Midlands are the single-storey rear extension, the side-return, the wrap-around, the double-storey addition and the over-garage extension. It is heated by your central heating, and you walk into it from inside.
That physical connection is the real difference. Everything else, from cost to planning to resale, follows from it.
Cost: where the savings are real and where they shrink
On headline figures a garden room wins. One 2026 comparison puts a traditional single-storey rear extension at £1,500 to £3,500 per square metre, with all-in costs of £35,000 to £70,000 for a 16 to 20 sqm build. A high-specification garden room of similar size is quoted at £17,500 to £28,000, a saving of roughly 30% to 50%. A separate guide gives an average of £20,000 to £50,000 for a fully finished garden room, at £1,500 to £2,200 per square metre.
Those numbers come from general UK guides, not from a quote for your house. Treat them as a range to test, not a price.
The costs people forget on a garden room
The sticker price of a garden room often excludes the work around it. Project managers point out that electrical trenching, data cabling, plumbing and groundworks can add £1,500 to £4,000, depending on how far the building sits from the house. A long garden in a Victorian Birmingham terrace can push you to the top of that range.
Add to your list:
- Ground clearance, levelling and drainage.
- A proper electrical supply from the consumer unit, with a certificate.
- Heating and insulation to a standard you can use in January.
- Landscaping to repair the garden after the build.
- Planning or building control fees if your design needs them.
The costs people forget on an extension
Extensions carry their own extras: structural engineer, party wall notices if you share a boundary, building control fees, a new kitchen or bathroom fit-out, flooring, decorating and moving drains. A fair budget includes a contingency. Our own guides on what a house extension costs per square metre and all-in UK extension prices by type and region break these items down.
Whichever route you choose, ask for an itemised estimate. Even a landscaping estimate example from a US contractor shows why line-by-line pricing beats a single headline figure: you can see what is included and compare quotes properly. The business is in Illinois, so use it for the format, not the prices.
A simple cost comparison
| Garden room | Single-storey extension | Garage conversion | |
|---|---|---|---|
| Typical published range | £1,500 to £2,200 per sqm (MyBuilder) | £1,500 to £3,500 per sqm (Cheshire Garden Rooms) | £625 to £1,250 per sqm (GetEstimateAI) |
| Hidden extras | Utility runs, groundworks | Fit-out, engineer, party wall | Replacing the garage door wall, floor level |
| Heated by | Own electric system, usually | Main heating | Main heating |
| Counts as house space | Generally not | Yes | Yes |
The garage figure comes from a 2026 UK cost guide, which also says a single integral garage conversion typically falls between £10,000 and £20,000. These are generalised estimates and vary by site.
Planning permission and building regulations
This is where the two options diverge most, and where mistakes are expensive. The rules below apply in England. Wales, Scotland and Northern Ireland differ.
Garden rooms
Many garden rooms are built under permitted development, so no planning application is needed. Which? sets out the limits: a maximum eaves height of 2.5m, an overall height of 4m for a dual-pitched roof (3m for other roofs), and 2.5m overall if the building is within 2m of a boundary. Balconies, verandas and raised platforms above 0.3m are not allowed. The building must be incidental to the house, not self-contained living accommodation.
The cumulative area catches people out. The government's technical guidance says buildings cannot cover more than 50% of the land around the original house. That total includes sheds, greenhouses, garages and earlier extensions, so a house that has already been extended has less room than you think. Check the Planning Portal's outbuilding rules too, because listed buildings, conservation areas and some designated land have tighter limits.
If you want a guest bedroom or a self-contained annexe, the answer changes. Experts note that a room used as sleeping or self-contained living space is a different thing in law. It often needs full planning permission and compliance with Building Regulations on drainage, fire safety and insulation. Ask your local planning authority, or apply for a lawful development certificate, before you order anything.
The "four-year rule" is out of date
People still say a building is safe from enforcement after four years. In England, since April 2024 the period is ten years for the change of use of a building to a single dwelling, so that shortcut no longer works. Do not build first and hope.
Extensions
Extensions come with a different rulebook. Some single-storey rear extensions fall under permitted development, with depth limits and a larger-home neighbour consultation route described in the government guidance. Two-storey and many side extensions need full planning permission. All of them need Building Regulations approval, because they are part of the house: structure, insulation, fire safety, drainage and ventilation. Our guide to side extension planning rules in the West Midlands covers the single-storey-under-4m situation in detail.
Build time and disruption
A garden room is quicker to install. Much of it is made off site, the footprint is small and your house stays intact. You may be using it within weeks of groundworks finishing, though your supplier's lead time is a separate matter.
An extension takes longer. Design, structural calculations, planning or prior approval, building control and the build itself all take time, and you live next to a building site during the work. The compensation is a result that looks and feels like it was always part of the house. A single team handling design through to handover, as we do, cuts the delays that come from chasing separate designers, planners and builders.
Resale value and mortgages
Both can help you sell, but not in the same way.
Market data from 2025 suggests a well-built garden room can add roughly 5% to 15% in appeal-driven value, with one report showing a net gain of about £8,000 after an average £12,000 installation cost. Another estimate has garden rooms adding about 7.5%, or around £20,000, on an average UK price. Those are estimates, and net gains swing with build cost.
Surveyors treat the two differently. Because a garden room is physically separate, it generally does not raise the bedroom count or internal floor area in valuation models. Its value is mostly buyer appeal, such as a home office or gym. An extension, by contrast, adds recorded floor area and can add a bedroom, which is what moves a house into a higher price bracket.
If your home is a three-bed semi in a street where four-bed houses sell for noticeably more, a double-storey or wrap-around extension changes your category. A garden room does not.
Which one fits your situation
Choose a garden room if
- You need a home office, studio, gym or hobby room and nothing more.
- The budget is tight and you can accept a building that is not connected to the house.
- You have a garden long enough to lose the space without losing the lawn.
- You want the least disruption inside the home.
Choose an extension if
- You want a larger kitchen-diner, a utility room or a family space that flows from the existing rooms.
- You need an extra bedroom or bathroom.
- You want to raise the value of the house in the eyes of a surveyor, not just a viewer.
- You want the new space heated and serviced like the rest of the house.
We have guides to the practical detail behind each of these: a kitchen extension, a utility room extension and a home office extension built as part of the house.
Garage conversion vs extension
If you have an integral or attached garage, a conversion competes with both options. It is usually the cheapest per square metre because the shell already exists: foundations, walls and roof are in place. You mainly pay for insulation, a floor, a new front wall in place of the door, electrics and heating. Building Regulations approval is required, and whether you also need planning permission depends on your property, so check with your council.
The trade-offs are real:
- You lose parking and storage, which matters in many Birmingham streets.
- The space is limited to the garage footprint, typically too small for a family kitchen-diner on its own.
- Floor levels and damp-proofing need careful handling.
There is a middle path. Building up over the garage adds a bedroom and keeps the ground floor in use. Joining the garage into a rear or wrap-around extension creates one large open space. We design both, and our over-garage extension guidance explains what to budget.
Our perspective
We build extensions across Birmingham, Solihull and the wider West Midlands, so we are open about our bias. We would still tell you to buy a garden room if all you need is a desk and a door that closes. It is not a lesser choice for that job.
But most of the homeowners who contact us are not short of a desk. They are short of a kitchen that fits the family, a bedroom for a growing child or a room that does not depend on an extension lead and a heater. That is a problem a separate building rarely solves, and it is the reason people ask us about staying put instead of moving.
Two things we say at the first conversation. First, get a realistic budget before anyone surveys the site, so the project is shaped to the money and not the other way round. Second, work out the planning and Building Regulations route at the start. It is cheaper to design around a rule than to discover it halfway through. Having one team responsible from design to handover means one person to call when something changes.
— Raja
Talk to us about your extension
If the answer for your house is an extension, we design and build single-storey, double-storey, kitchen, rear, side-return, wrap-around and over-garage extensions, with planning, building regulations, budgeting and construction handled by one team. Tell us what you want the space to do and we will give you straight guidance on cost and route before you commit. Start at Extension Kings.
FAQ
Is a garden room cheaper than an extension?
Usually, yes. Published 2026 comparisons show a saving of roughly 30% to 50% for a similar size. The gap narrows once you add groundworks, utility connections and landscaping, and a garden room gives you less usable house space for the money.
Does a garden room add value to a house?
It can add buyer appeal, with estimates ranging from 5% to 15%, but surveyors generally do not count it as habitable floor area or an extra bedroom. An extension is more likely to be recognised in a valuation because it adds formal living space.
How high can I build in my garden without planning permission?
In England, under permitted development, an outbuilding can generally have 2.5m eaves and be up to 4m overall with a dual-pitched roof, or 3m with other roof types. Within 2m of a boundary the limit is 2.5m. Different rules apply in conservation areas and for listed buildings, so check with your council first.
Is a garage conversion better than an extension?
It is cheaper if you can give up the garage and the footprint is big enough for your needs. An extension or an over-garage addition is better when you need more floor area, an extra bedroom or a larger kitchen-diner. Both need Building Regulations approval.
Sources
- Do you need planning permission for a garden room? (Which?)
- Permitted development rights for householders: technical guidance (GOV.UK)
- Planning permission for outbuildings (Planning Portal)
- Planning services updates (Wealden District Council)
- Garden room extension cost guide 2026 (MyBuilder)
- Garden room vs extension cost (Cheshire Garden Rooms)
- Which garden improvements give the biggest boost in value (Property Reporter)
- Does a garden office add value to your property? (IPSE)
- Garage conversion cost UK 2026 (GetEstimateAI)