Quick answer: a conservatory or porch is exempt from the energy efficiency requirements only while all five of these hold: it is at ground level; its floor area is no more than 30 m²; the glazing meets Part K; it is thermally separated from the house by the existing walls, windows and doors (kept, or replaced with ones that comply); and it has no heating — the house's system is not extended into it and it has no fixed heater of its own. Break one and it is an ordinary extension. Other parts of the Building Regulations — structure, drainage, electrics, glazing safety — can still apply either way.
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The five conditions
- It is at ground level.
A conservatory over a garage or on a flat roof is an extension, whatever it is called. - Its floor area is no more than 30 m².
Measured internally. One square metre over and the exemption is gone entirely — there is no partial version. - The glazing satisfies Part K.
Safety glazing in critical locations — low-level panes, doors and the areas beside them. - It is thermally separated from the house.
The external walls, windows and doors between the house and the conservatory are retained, or replaced with walls, windows and doors that meet the energy efficiency requirements. Knock the back wall through and you have lost it. - It has no heating.
The dwelling’s heating system is not extended into it, and it has no fixed heating appliance of its own. A radiator off the boiler, a wet underfloor loop or a fixed electric panel all end the exemption. A plug-in portable heater does not.
First, is it actually a conservatory?
Before the exemption is even in play, the structure has to be substantially glazed: at least 75% of the roof and at least 50% of the external walls translucent. This is where orangeries and garden rooms fall out. A room with a solid insulated roof, a lantern in the middle and brick perimeter walls is an extension, whatever the brochure calls it — the 25% opening allowance and Table 4.2 apply to it in full.
What happens when a condition fails
It becomes an extension for Part L purposes, and three things follow. Its glazed and opaque elements must meet the Table 4.2 limiting values — windows and doors 1.4, rooflights 2.2, any solid wall 0.18, roof 0.15, floor 0.18 W/m²K. Effective thermal separation between the heated house and the new space is expected. And any heating in it needs its own temperature and on/off controls, independent of the rest of the house.
What does not follow is the opening-area limit. A conservatory that loses its exemption is not failed for being made of glass — the 25% rule is an extension provision. GlazeMonkey applies that distinction automatically: tell it the space is a conservatory and it stops applying the allowance, but keeps checking every element.
The two that catch people out
Heating. Running a radiator off the boiler is the usual one, and it is often done after completion — the conservatory is cold in February, a plumber adds a rad, and the exemption quietly ends. A plug-in portable heater does not count; anything fixed does.
The doors. Removing the patio doors to open the conservatory into the kitchen ends the thermal separation, and with it the exemption. It also tends to surface at the worst moment — a buyer's solicitor asking for the building regulations paperwork years later. If the plan is an open-plan space, design it as an extension from the start; at that point you are into the area-weighted calculation, and a mostly-glazed room needs a very good specification to pass it.
Frequently asked questions
Is a conservatory exempt from building regulations?
From the energy efficiency requirements, yes — but only while all five conditions hold: at ground level, no more than 30 m², Part K glazing, thermal separation from the house retained, and no heating of any kind. Break one and it is an ordinary extension, subject to the Part L element standards.
What counts as a conservatory in the first place?
To be treated as a conservatory rather than an extension, it has to be substantially glazed: at least 75% of the roof and at least 50% of the external walls translucent. An orangery with a modest roof lantern and solid perimeter walls does not qualify — it is an extension from the start.
Can I put a radiator in my conservatory?
Not without losing the exemption. Extending the house’s heating into the conservatory, or fitting any fixed heating appliance, makes it a heated part of the dwelling — so it must comply as an extension, and its own glazed and opaque elements must meet the Table 4.2 limiting values. That is a hard test for something made almost entirely of glass.
What if I take the patio doors out to open it up to the kitchen?
Then the thermal separation has gone and so has the exemption. This is the single most common way a compliant conservatory becomes a non-compliant extension, usually years after it was built and often discovered at sale. If you want it open to the house, design it as an extension from the outset.
Does the 25% glazing rule apply to a conservatory?
No. The opening-area limit applies to extensions. A conservatory or porch that loses its exemption still has to meet the Table 4.2 U-values for its glazed and opaque elements, and needs effective thermal separation and its own heating controls — but it is not failed simply for being mostly glass. GlazeMonkey applies that distinction for you.
What about a porch?
The same five conditions, minus the "substantially glazed" definition — a solid porch is still a porch. Ground level, no more than 30 m², Part K glazing, the existing front door retained in place, and no heating extended into it.
Next: the U-values an extension must meet · the area-weighted calculation · when a whole-dwelling SAP is the answer.
Basis: Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments (England), Section 10 and the exemption in Regulation 9 / Schedule 2.