Conservatory vs extension: cost compared
A conservatory is the budget route to extra space: typically about £14,000, versus roughly £40,000 to £64,000 for a 20 m² single-storey extension before fit-out. An extension is fully integrated, better-insulated living space and usually adds more value, while a conservatory is cheaper and quicker but less usable in deep winter unless it has a solid roof.
Conservatory vs House extension: the cost
| Option | Range | Typical | Priced |
|---|---|---|---|
| Conservatory | £5,000 to £35,000 | £14,000 | installed |
| House extension | £2,000 to £3,200 | £2,500 | per m² |
Costwise guide prices, reviewed June 2026, not quotes. Note the two are quoted on different bases, so the figures above are not a like-for-like comparison on their own.
At the same size
A conservatory is quoted as a whole job; an extension is quoted per square metre. At the same 20 m² footprint the gap is the honest comparison.
| Option | How it is priced | Typical at 20 m² |
|---|---|---|
| Conservatory | whole job, size does not change the quote the same way | £14,000 |
| House extension | £2,500 per m² x 20 | £50,000 |
Conservatory vs House extension: the difference
| Factor | How they compare |
|---|---|
| Typical cost | Conservatory about £14,000 installed; extension about £2,000 to £3,200 per m² (a 20 m² build is about £40,000 to £64,000). |
| Comfort and use | An extension is insulated, year-round living space. A glazed conservatory can be cold in winter and hot in summer unless you add a solid or tiled roof. |
| Value and planning | An extension typically adds more to the property value; conservatories are more often permitted development within size limits. |
| Best for | Conservatory: extra space on a budget, fast. Extension: proper, year-round living space and more added value. |
Planning permission treats a conservatory as an extension
A conservatory does not get an easy ride from the planning rules. Both are judged under the same permitted development class, so what decides it is size and position, not what you call the room. In England a single-storey rear extension can project up to 4 metres beyond the rear wall of the original house if the house is detached, or 3 metres for any other house, and cannot exceed 4 metres in height. Within 2 metres of a boundary the eaves cannot exceed 3 metres. Across the plot, only half the land around the original house can be covered by extensions or other buildings. A prior approval application raises the projection limit to 8 metres detached and 6 metres otherwise, but only where the house is not on designated land such as a conservation area. The devolved nations set their own limits.
A small conservatory at the back of the house usually clears all of that. Put it on the front, or build in a conservation area, and it can need permission whatever its size. The extension most people actually want is far more likely to touch the limits, which is a difference in time and paperwork rather than money. Choose a conservatory because the space and the budget suit it, not to avoid the planning system.
Building regulations are where the two genuinely part company
This is the real divide, and it is where the price gap comes from. A conservatory is normally exempt from building regulations if it is single storey, at ground level, less than 30 square metres in floor area, separated from the house by external quality walls, doors or windows, heated by its own system with separate temperature and on/off controls, and glazed and wired to the applicable standards. Miss one of those and the exemption goes.
Most extensions need approval, and it reaches the foundations, floors, external walls, roof, drainage, ventilation, electrics and energy efficiency. An extension is therefore built to the standard of the rest of the house, and a conservatory is not required to be. That difference in standard, rather than the paperwork itself, is what costs the money.
One trap to know early: a new structural opening between an exempt conservatory and the house still needs approval. If the plan is to take the doors out and make one open room, the exemption goes, and you are paying towards extension standards for conservatory-grade walls. If that is the intention from the start, price the extension properly before you commit.
The roof and the heating decide whether you use it in January
A glazed conservatory runs cold in winter and hot in summer, and no energy efficiency standard applies to an exempt structure, so nothing obliges it to do better. Changing the roof is the change that makes the most difference. A solid or tiled roof instead of glass adds £5,000 to £9,000, lifting a typical £14,000 conservatory towards £20,000 to £25,000 depending on size, still well under the £50,000 a 20 square metre extension costs at the mid rate.
Two conditions come attached. A new roof has to comply with building regulations to be structurally safe, energy efficient and properly ventilated, so the existing frames and base need to be checked as able to carry the extra weight. And because the exemption requires independent heating with its own controls, running the main central heating loop into the room and leaving it on the house thermostat ends it. An electric radiator or a circuit with its own controls is the usual answer.
Build time, disruption and the costs outside the headline figure
A conservatory is 2 to 5 weeks on site. A single-storey extension is 10 to 16 weeks, and that is the build, not the drawings, the planning decision or the building control visits. The extension also means foundations, an opening cut through an external wall and plaster that has to dry, so if the new room is your kitchen, plan for a stretch without one.
The per square metre rate covers the shell and a basic finish, not the finished room. Design and drawings run at roughly 7 to 15 per cent of the build cost. A party wall surveyor is around £700 to £1,500 per neighbour if you build close to a boundary. A fitted kitchen is £10,000 to £25,000 and bi-fold doors £4,000 to £12,000. A conservatory is quoted as a whole job installed, so the number you are given sits closer to what you end up paying, and for some people that predictability matters as much as the space.
Which one to build
Build the conservatory if the budget is nearer £14,000 than £50,000, you want the room within a couple of months, and it can work as a separate space reached through a door. If you plan to use it all year, price the solid roof in from the start rather than treating it as a later upgrade.
Build the extension if the room has to be part of the house, if it needs to hold a kitchen, bathroom or bedroom, or if you already know you will knock through, because the exemption that makes a conservatory cheap disappears the moment you do.
Two things sharpen the choice. In London and the South East a conservatory carries a 15 to 25 per cent premium while an extension per square metre runs 20 to 40 per cent above the national rate, so the gap widens rather than narrows. On resale an extension typically adds more than a conservatory, because it adds insulated floor area inside the house where a conservatory reads as an add-on. Neither is guaranteed to return what it cost, and over-extending for the street can cap the gain.
Common questions
Yes, usually by a wide margin. A typical conservatory is around £14,000 installed, while a single-storey extension of similar floor area is often £40,000 or more before any kitchen or bathroom fit-out.
Generally no. A well-built extension adds insulated, integrated living space that tends to add more value, whereas a basic conservatory adds less, especially if it is not usable all year.
Usually not. A conservatory is normally exempt if it is single storey, at ground level, less than 30 square metres in floor area, separated from the house by external quality walls, doors or windows, and heated by its own system with separate temperature and on/off controls, with compliant glazing and fixed electrics. Most extensions do need approval, and it covers foundations, floors, external walls, roof, drainage, ventilation, electrics and energy efficiency. That difference in standard is a large part of why about £14,000 buys a conservatory while a 20 square metre extension is about £50,000 at the mid rate.
Not without care. One condition of the building regulations exemption is an independent heating system with separate temperature and on/off controls, so extending the main loop into the room and leaving it on the house thermostat ends the exemption. Most people fit an electric radiator instead, or a circuit with its own controls, and because a lot turns on what counts as independent it is worth confirming your arrangement with the local building control team before it goes in. An extension has no such condition, because it is designed and approved as part of the house.
You can, but it changes the job. Any new structural opening between the conservatory and the existing house needs building regulations approval even when the conservatory itself is exempt, and taking out the external quality doors or windows that separate the two ends the exemption. Once the room is open to the house the extension rules apply to it instead, and that is the point at which a £14,000 conservatory stops being an alternative to a £50,000 extension and becomes a cheaper version of one.
Yes, by a wide margin. A solid or tiled roof instead of glass adds £5,000 to £9,000, so a typical £14,000 conservatory lands towards £20,000 to £25,000 depending on size, against about £50,000 for a 20 square metre extension at the £2,500 per square metre mid rate. A solid roof holds heat better than glass, which is what makes the room usable through the winter. It is still a conservatory, though: separated from the house, on its own heating, and not held to the energy efficiency standard an extension has to meet.
A conservatory takes 2 to 5 weeks on site depending on size and roof type, while a single-storey extension takes 10 to 16 weeks. Set the two ranges against each other and the extension is at least twice as long, and can be several times longer. Its timeline also sits behind design work, a planning decision where permitted development does not cover the size, and building control inspections, so the gap between deciding to go ahead and using the room is wider than the build figures alone suggest.
These are independent guide prices based on typical UK jobs in 2026. Your actual cost depends on your property, spec, access and where you live. Always get at least three written quotes before committing.