Conservatory or extension: which is right for your house?
The honest answer is that planning barely separates them — both are treated as extensions and both can be permitted development. What actually differs is Building Regulations: a conservatory can be exempt, an extension never is. That single fact shapes the cost, the comfort and the value you get.
The short answer
If you want a bright, part-time garden room for spring to autumn and you want to keep the build cheap and simple, a conservatory is hard to beat — kept glazed and separately heated, it sidesteps full Building Regulations altogether. If you want a room you can live in every day of the year, that counts as proper floor area when you sell, and that ties into the rest of the house, you want an extension and you should budget for the regulations that come with it.
Most people who tell me they “can’t decide” are really deciding between cheap-but-seasonal and dearer-but-permanent. Get that trade-off straight and the rest follows.
Planning treats them the same
A conservatory counts as an extension for planning purposes, so the same permitted development limits apply to both. Single storey, at the rear, and you get up to 4m of depth on a detached house or 3m on a semi or terrace, with a 4m height cap — or up to 8m and 6m respectively through the larger home extension prior approval route. Stay inside those limits, in matching-ish materials, and neither a conservatory nor a modest extension normally needs a full planning application.
So planning is rarely the thing that decides it. If your plot, your boundaries or a conservation-area designation rules out one, it usually rules out the other too. For the full set of limits, see our permitted development rules. Where a householder application is needed, the fee in England is £548 from 1 April 2026, and that figure is the same whichever you build.
Where they really differ: Building Regulations
This is the real fork in the road. A conservatory is exempt from Building Regulations only if it meets every one of a specific set of conditions. Miss any one and it is an extension in the eyes of the law, with the fabric, insulation and structural standards that go with that.
The Planning Portal sets out the exemption like this: the structure is built at ground level and is under 30m² in floor area; it is single storey; it is separated from the house by external-quality walls, doors or windows; it has an independent heating system with its own on/off and temperature controls; and its glazing and any fixed electrics meet the relevant Building Regulations. An extension meets none of that exemption — it has to comply in full, always.
One point catches people out: any new structural opening you form between the house and the conservatory needs Building Regulations sign-off even when the conservatory itself is exempt. Knocking a hole in a load-bearing wall is controlled work regardless of what sits on the other side of it. Our guide to Building Regulations for extensions covers what that approval involves.
Conservatory or extension, side by side
| Conservatory (kept exempt) | Extension | |
|---|---|---|
| Planning permission | Same PD limits: 4m / 3m rear depth | Same PD limits: 4m / 3m rear depth |
| Building Regulations | Exempt if all conditions met | Full approval always required |
| Floor area cap for exemption | Under 30m², single storey | No cap; must comply whatever the size |
| Roof | Mostly glazed or translucent | Solid, insulated to current standards |
| Doors to the house | External-quality, kept in place | Open through; no thermal separation needed |
| Heating | Independent, with its own controls | Can run off the main house system |
| Counts as floor area in an EPC | No — excluded while exempt | Yes — and must meet fabric standards |
| Best for | Seasonal garden room, lowest cost | Everyday living space, resale floor area |
Exemption conditions and permitted development limits from the Planning Portal (conservatories) and GOV.UK Permitted Development technical guidance; householder application fee from the Planning Portal fee schedule effective 1 April 2026.
When a conservatory quietly becomes an extension
The exemption is a live thing, not a one-off status. You can build a perfectly exempt conservatory and then lose that exemption later with a change that seems cosmetic. Three do it most often.
Swap the glazed or polycarbonate roof for a solid tiled one and you have almost certainly crossed the line — the roof is no longer mostly translucent, and the added weight often needs the base and frame checked by a structural engineer. Take out the external-quality doors to open the space into your kitchen, and you have removed the thermal separation the exemption depends on, so the whole space now has to meet the insulation and heat-loss rules an extension must meet. Tie the conservatory into the main heating circuit rather than heating it independently, and you have done the same thing by a different route.
None of this is against the rules. It just means the work becomes controlled, and it should be signed off. The trap is doing it informally, then discovering at sale that a buyer’s solicitor wants a completion certificate you never got. If you think you will eventually want a solid roof or an open-plan link, it is usually cheaper and cleaner to build it as an extension from the start.
Which one suits your house
Think about how you will actually use the room. A glazed conservatory is bright and cheap to add, but it runs hot in July and cold in January unless you spend on high-performance glass, blinds and heating — at which point some of the cost advantage has gone. An extension is comfortable year-round by design, because it is built to the same fabric standards as the rest of the house.
Value tends to follow that same line. A well-built extension usually adds measurable resale value because it adds insulated floor area that counts, and Nationwide’s house-price research has long put the gain from adding around a tenth to a home’s floor space at roughly 5%, more where it adds a proper bedroom. A conservatory is more mixed: a smart, warm one is a genuine selling point, while a tired, freezing one can put buyers off. Because an exempt conservatory is left out of the EPC calculation, it neither helps nor hurts your energy rating; an extension is inside it and has to pull its weight.
On cost, a glazed conservatory is usually the cheapest way to gain a room — broadly in the region of £10,000 to £25,000 for a typical size, where a brick single-storey extension of the same footprint usually costs a good deal more. Those ranges come from BestBuilders’ 2026 regional cost model (published national cost guides × a regional labour index, averaged across 519 UK towns) and are indicative, not quotes; for a full breakdown by style, size and roof, see BestBuilders’ conservatory cost guide.
So the decision rule is simple enough: choose the conservatory if you want light and low cost and you can live with a seasonal room; choose the extension if you want a permanent, everyday space that counts when you sell — and remember that the moment you make a conservatory work all year, you have effectively paid for an extension anyway.
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