Orangery extension

Three things. The roof is mostly solid: a flat deck, insulated and plastered underneath, with one or two glass lanterns rising out of it. The walls have masonry in them, as corner piers, dwarf walls or full-height brick panels between the windows. And the glazing is tall, usually running from a low sill or the floor up to a deep fascia.

Neither planning law nor the building regulations use the word. To a council an orangery is a single-storey extension, and the same limits apply to it as to any other. The name matters to the people selling it, and to you only as shorthand for how the room will look and feel.

Expect £850 to £3,000 per m² of floor in 2026, which puts a 20 m² room at £17,000 to £60,000 depending on the frames. The table sets that beside a conventional single-storey extension of the same size.

These are national figures with VAT included; the orangery cost calculator gives a range for your own size and frame.

This decision changes the cost more than the choice of frame. Keep the existing back doors and the room needs no steel, no propping and no new opening, and your heating stays as it is. It still needs building regulations approval: the conservatory exemption only covers rooms with at least three quarters of the roof glazed, and an orangery deck is mostly solid.

Take the doors and the wall out and the job grows. You will need a structural design and a steel beam for the new opening, temporary propping, and making good inside the house. Open or closed, the energy rules set a glazing allowance of 25% of the new floor, topped up by whatever window and door area the new room blocks off. Most orangeries go over it, which is allowed if a heat-loss calculation proves the room performs no worse than a compliant one, usually by using better glass and more insulation in the deck.

None of this should put you off an open-plan room. It is the version most people want. Budget for it at the start, because a builder who prices a closed-off room and is then asked to knock through will come back with a larger number.

Many orangeries in England are built under permitted development, with no application. The main tests: one storey, a maximum height of 4 m, a depth of up to 4 m on a detached house and 3 m on others, and eaves of 3 m or less within 2 m of a boundary. All extensions and outbuildings together may take up no more than half the garden around the original house.

Deeper rear rooms, to 8 m on a detached house or 6 m on other houses, go through a prior approval process in which the council consults the neighbours. Anything beyond that needs a full householder application. In England the council fee from 1 April 2026 is £548 for a householder application and £249 for prior approval. A lawful development certificate is optional, but useful when you sell.

Flats and maisonettes have no permitted development rights. Listed buildings need listed building consent whatever the size. In a conservation area, a National Park or a National Landscape, side extensions are excluded and the larger rear allowance does not apply. Wales follows a similar pattern without the larger allowance. Scotland has its own classes, and an orangery there almost always needs a building warrant as well.

A typical job is on site for eight to twelve weeks. Foundations and the floor slab take the first fortnight, longer if building control asks for deeper trenches near trees. Brickwork follows, then the roof structure and its waterproof covering, at which point the room is dry even without glass. Frames, doors and the lantern go in over a few days. Plastering, electrics, heating and the floor finish take up the final weeks.

If the room is to be open to the house, the opening is normally formed last, once the new room is weathertight. You keep a closed back wall for most of the job and lose it for a week or two.

A quote is only as good as the information behind it. Five things make the difference between a firm price and a guess.

• A rough width and projection, and which wall of the house it joins.
• Whether the room will be open to the house or kept behind doors.
• Where the drains run. A manhole in the patio often means a public sewer nearby, and building within 3 m of one needs the water company’s agreement.
• Any trees within about 10 m, and whether the ground is clay.
• What the house is: listed, in a conservation area, a new-build with planning conditions, or none of those.