Conservatory to Extension Conversion: Cost and Building Regs
Converting a conservatory into a habitable room costs £10,000–£35,000 and ends its building regs exemption. The Part L U-values and quote lines to check.
What does it cost to convert a conservatory into a proper room?
Converting an existing conservatory into a habitable room costs roughly £10,000–£35,000 in 2026, depending on size and how much of the original structure survives. A solid insulated roof on its own is £4,000–£12,000. Demolishing the conservatory and rebuilding as a conventional extension is a different project at £25,000–£65,000. But the number that decides your quote is not on the price list. Class 7 of Schedule 2 to the Building Regulations 2010 exempts a conservatory on three structural conditions only: ground level, a floor area not exceeding 30m², and glazing that satisfies Part K. What ends the energy exemption is regulation 21(4), which disapplies the energy efficiency requirements only while the separating wall, door or window is still there, the house heating has not been extended in, and no fixed appliance has been fitted to heat the space. And under regulation 9(1)(b), the work stays exempt only if the finished structure is still a conservatory of the kind Schedule 2 describes. Convert it and it is not, so it has to meet the extension standards in Approved Document L. A quote that does not say how it will do that is not a quote for this job.
Costs based on BCIS regional benchmarks, supplier pricing for solid roof systems, and MyBuildAlly quote analysis, 2026. London and the South East typically run 15% to 30% above these figures. Your actual costs will vary with the condition of the existing base and how much glazing you keep.
Why a conservatory is exempt in the first place
A conservatory is not exempt because it is a conservatory. It is exempt because it meets a specific list of conditions, short enough to check yourself. Approved Document L rolls the three Class 7 conditions and the regulation 21(4) triggers into a single list of five.
Where a dwelling is extended by adding a conservatory or porch, the work is exempt from the energy efficiency requirements, under regulation 21 of the Building Regulations, if all of the following apply. a. The extension is at ground level. b. The floor area of the extension does not exceed 30m². c. The glazing complies with Part K of Schedule 1 to the Building Regulations. d. Any wall, door or window that separates the extension from the dwelling has been retained or, if removed, has been replaced with a wall, door or window. e. The heating system of the dwelling is not extended into the conservatory or porch.
Two of those five conditions are the ones a conversion breaks. The separating wall, door or window is the thermal separation between a cold glass box and your heated house. Take it out and the box becomes part of the house. Run a radiator or an underfloor loop off the house system into it and you have done the same thing by a different route. So has fitting a fixed heating appliance of the conservatory's own, a hard-wired panel heater or a log burner, which regulation 21(4)(c) treats as a separate trigger in its own right. A plug-in oil-filled radiator is not a fixed appliance. Anything wired in or flued is.
You will still see installers quote a "75% of the roof and 50% of the walls must be glazed" test. That split comes from older guidance and from Planning Portal descriptions of what a conservatory is. It is not one of the exemption conditions in the current Approved Document L, and a building control body applies the five conditions above instead.
What changes the moment the exemption goes
Approved Document L is blunt about it. A conservatory must keep thermal separation from the dwelling. Remove it, or extend the house heating into the space, and the conservatory is treated as an extension with the extension rules applying in full.
That means new thermal elements have to hit the limiting U-values for new fabric in an existing dwelling:
| Element | Maximum U-value (W/m²K) |
|---|---|
| Roof | 0.15 |
| Wall | 0.18 |
| Floor | 0.18 |
| Window | 1.4, or Window Energy Rating Band B |
| Rooflight | 2.2 |
| Door with over 60% of the internal face glazed | 1.4, or Doorset Energy Rating Band C |
| Other doors | 1.4, or Doorset Energy Rating Band B |
These come from Table 4.2 of Approved Document L Volume 1. They are worth writing into your quote verbatim, because they are unusually stable: the 2026 edition, which takes effect on 24 March 2027 in England for building work not connected with higher-risk buildings and on 24 September 2027 where it is, carries exactly the same figures for new fabric elements in existing dwellings. It does renumber them, so from those dates Table 4.2 becomes Table 3.2 and Table 4.3 becomes Table 3.3. And the Future Homes Standard, which shares the first date, applies to new dwellings rather than extensions, so nothing about your conversion changes because of it.
Those figures apply to new elements. Existing fabric that becomes a thermal element because of the conversion, meaning the retained dwarf wall and the old floor slab, falls under paragraph 10.7e and Table 4.3 instead. The improved-column targets there are 0.30 W/m²K for a wall insulated internally or externally, 0.25 for a floor and 0.16 for a roof. That is materially easier than 0.18, and it is the difference between lining the dwarf wall and rebuilding it. A quote that prices every surface to 0.18 has not read the table, or is quoting a rebuild.
A retained glazed element is not treated like a new one. Replacing the glass inside an existing frame is not "providing a controlled fitting" under Approved Document L, so it does not itself have to reach 1.4 W/m²K. Supplying a complete new window or door unit does. The cheap version of a conversion exploits that, keeping the old frames and swapping the units.
The rest of the building regulations arrive at the same time. Part A for the structure now carrying a heavy roof, Part C for damp and the floor build-up, and Part F for ventilation of a habitable room. Part P for the electrics applied all along: regulation 9(3) extends Part P to any Class 7 extension taking its electricity from the house, exempt or not, so that work needed certifying before the conversion as well as after it. Your builder should be naming a building control route, either the council or an approved inspector, before work starts rather than after.
The glazing area rule is the one that catches conservatories
This is the part that surprises people, and the reason a straight "make it warm" conversion often will not pass as designed.
The total area of windows, roof windows, rooflights and doors in extensions should not exceed the sum of the following. i. 25% of the floor area of the extension. ii. The total area of any windows and doors which no longer exist or are no longer exposed due to the extension.
Work it through on a typical 16m² conservatory. Twenty-five per cent of the floor area gives you 4m². Add the old patio doors and kitchen window the conservatory now covers, say another 5m², and the allowance is about 9m². Full-height glazing on three sides is comfortably past that before you count the roof. The cap only bites where paragraph 10.12 applies, meaning the separation has gone or the heating has been extended in. A conservatory that keeps its doors and is non-exempt for another reason, most obviously one over 30m², is taken out of the cap by paragraph 10.13c.
Two lawful ways round it exist, and both are calculations rather than opinions. The first is area-weighted: show that the area-weighted U-value of the converted space is no worse than a compliant extension of the same size and shape. The second is a Standard Assessment Procedure calculation against the same dwelling plus a compliant notional extension. That is what lets you keep a lot of glass, because you buy the glazing area back with a much better roof, walls and floor than the minimum.
Neither route is free, and the sums are usually the designer's or an energy assessor's. If your quote keeps the glazing and says nothing about how it demonstrates compliance, the calculation has not been done.
Ask which of the three compliance routes the contractor is using: element-by-element against Table 4.2 with the glazing area capped, the area-weighted U-value comparison, or a SAP calculation against a notional extension. Ask who produces the calculation and whether the fee is in the price. If the answer is a shrug, the quote is not priced for a compliant job.
The structure underneath is where the money hides
A conservatory base was designed to carry aluminium or uPVC frames and a light translucent roof. It was not designed to carry a tiled roof, insulated walls and a plastered ceiling. The quotes we see are consistent on the point: this budget is won or lost underground.
Conservatory bases are commonly a shallow strip footing or a thin raft, well short of the minimum foundation depth a habitable extension needs. The dwarf wall is often a single skin. Neither is automatically a problem, and neither can be assumed. Only a structural engineer's inspection of the base and wall tells you, and that report is what moves you from the bottom of the price range to the top. Structural engineer fees for a survey and assessment of this kind run £500–£1,000, which is the cheapest insurance in the project.
The same applies to the roof. Lightweight tiled and panel systems are sold on being lighter than clay tiles, which is true and irrelevant to whether your frame can carry them. A reputable supplier provides structural calculations and wants to know what is underneath. One that quotes off a photo has not checked.
What the work costs, line by line
| Item | Typical 2026 cost |
|---|---|
| Structural survey and engineer's calculations | £500–£1,000 |
| Foundation or base strengthening, where needed | £2,000–£8,000 |
| Solid insulated roof, supplied and fitted | £4,000–£12,000 |
| Rebuilding or upgrading walls | £2,500–£15,000 |
| Insulated floor build-up and screed | £2,000–£6,000 |
| Replacement glazing and doors | £3,000–£10,000 |
| Extending heating into the space | £800–£3,500 |
| Electrical work, Part P certified | £600–£2,500 |
| Plastering and finishes | £2,000–£6,000 |
| Building control application | £500–£1,500 |
Those line items are why the bottom of the range is £10,000 and not lower. Strip the table back to what no conversion can avoid, at every minimum, and the survey, roof, floor, heating, electrics, plastering and building control fee reach about £10,000 before a course of blockwork or a new glazed unit.
Put together by size, the routes look like this:
| Conservatory size | Full conversion to a habitable room | Demolish and rebuild as an extension |
|---|---|---|
| Up to 12m² | £10,000–£16,000 | £25,000–£40,000 |
| 12–20m² | £15,000–£24,000 | £30,000–£50,000 |
| 20–30m² | £22,000–£35,000 | £40,000–£65,000 |
The right-hand column is not a conversion and should not be presented as one. If the engineer condemns the base, you are building a new extension on the same footprint, and it should be quoted and compared as such against our kitchen extension cost breakdown. Conversion is cheaper only when the existing structure genuinely survives the inspection.
Does it need planning permission?
Usually not. The conservatory already exists, and re-roofing or insulating it normally sits within permitted development for a house, as long as the completed structure still meets the height limits after a taller roof goes on. That last point catches people: a pitched tiled roof can be noticeably higher than the polycarbonate it replaced.
Permission is needed if the property is listed, in a conservation area, a flat or maisonette, or covered by an Article 4 direction that has removed permitted development rights. Our planning permission guide covers the thresholds. If you want certainty for a future sale, a Lawful Development Certificate from the council gives you written confirmation that the work was permitted development.
Common mistakes
Assuming the exemption survives the knock-through. The most common version of this conversion is taking out the French doors between the kitchen and the conservatory to make one open space. That single act removes the thermal separation and turns an exempt structure into a notifiable extension. It is usually done as a finishing touch at the end of a job that avoided building control throughout, and the problem surfaces when a buyer's solicitor asks for a completion certificate.
Treating the 30m² limit as the whole test. Floor area is one condition of several, and the regulation says "does not exceed 30m²", not "under". Plenty of conservatories are 12m² and still lose their exemption on the heating or the separating doors. An installer who answers a building regulations question by citing the 30m² limit alone has answered a different question.
Ordering the roof before anyone has looked at the base. Solid roof systems are sold hard and fast, and the structural check is the step that gets skipped because it delays the sale. If the base or dwarf wall turns out to be inadequate after the old roof has come off, you are negotiating from the worst possible position. The engineer's report costs £500–£1,000 and belongs at the start.
Accepting a quote with no U-values and no glazing calculation. A conversion quote should name the target U-values for roof, walls and floor, the insulation product and thickness, and how the glazing area is justified. If none of that appears, the quote is priced for a job that will not pass, and the shortfall becomes a variation later. This is the commonest form of a quote with the building regs left out.
Comparing a conversion quote against a rebuild quote. They are different products: one keeps your base and frame, the other replaces them. A £14,000 conversion looking cheap next to a £34,000 extension is no saving if the engineer has said the base will not carry a solid roof. Make every quote state in writing what it assumes about the existing structure.
Check the quote before you commit
The strength of a conservatory conversion quote is not the headline figure. It is whether the structural report, the U-values, the glazing compliance route and the building control fee are actually in there, or have been left out to make the number look competitive against the installer down the road.
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- The Building Regulations 2010, Schedule 2: Exempt buildings and work - Class 7, the structural conditions for exemption
- The Building Regulations 2010, regulation 9 - exempt buildings and work; reg 9(1)(b) and reg 9(3) on Part P
- The Building Regulations 2010, regulation 21 - application of the energy efficiency requirements; reg 21(4)(a) to (c)
- Approved Document L, Conservation of fuel and power, Volume 1: Dwellings - 2021 edition incorporating 2023 amendments, paragraphs 0.14, 10.6, 10.7 including 10.7d and 10.7e, 10.9, 10.10, 10.12, 10.13, and Tables 4.2 and 4.3, plus the 2026 edition, Table 3.2, taking effect 24 March 2027 for building work not connected with higher-risk buildings
- Planning Portal: Conservatories, building regulations - exemption conditions and structural openings
- BCIS Average Building Prices - regional cost benchmarks, 2026, plus analysis of conservatory conversion quotes submitted to MyBuildAlly
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