3m x 4m Conservatory Cost UK 2026: £11,000 to £18,000

Why the 3m x 4m Orientation Matters

Most quote websites and most cost calculators treat 3x4 and 4x3 as the same job. From a costing perspective they are within a few hundred pounds of each other. From a planning perspective and a usability perspective they are meaningfully different, and getting the orientation wrong leads to one of the most common owner regrets: a room that looks awkward against the house, blocks an existing window or door, or eats more garden than expected.

The 3 metre wall against the house suits properties where the rear elevation has limited clear span: a Victorian terrace with a back door and kitchen window flanking it, a 1930s semi with a galley kitchen and a separate utility door. Trying to fit a 4 metre wall in those scenarios means either blocking an existing opening (costly to reinstate, may need building control involvement) or pushing the conservatory off-centre so it sits awkwardly under one corner of the property.

The 4 metre projection into the garden is the more dramatic feature. From inside, you get a much longer view down the garden than you would from a 3 metre projection. From outside, the conservatory becomes a meaningful garden feature in its own right rather than a small bump on the back of the house. The trade-off is that 4 metres of projection on an attached house exceeds the 3 metre standard permitted development limit, triggering the larger home extension prior approval notification process. This is not planning permission proper, but it does require a 42-day notification to the council with neighbour consultation. Most of these go through without issue.

Planning Quirks Specific to 3m x 4m

Two planning details are specific to the 3m x 4m orientation where the 4 metre dimension projects into the garden. First, the prior approval notification. Under the larger home extension permitted development rules, an attached house can extend up to 6 metres beyond the rear wall (8 metres for a detached) but anything beyond 3 metres needs prior approval. The council notifies your neighbours, takes objections for 21 days, and then has another 21 days to issue a decision. Most decisions are positive. Fees are roughly £96 in England as of 2026 (check Planning Portal).

Second, eaves and ridge height restrictions. Under PD a single-storey rear extension cannot exceed 3 metres at the eaves where it is within 2 metres of a boundary, and the overall ridge height cannot exceed 4 metres anywhere. A long, deep conservatory with a steeply pitched roof can hit these limits on certain plot configurations. Always verify the eaves height against your boundary distance before signing a quote.

Where the 3 metre wall is against the house (so only 3 metres of projection), the standard PD rules apply with no prior approval needed. This is the simpler regulatory route and is the reason many installers default-design to that orientation regardless of whether it is the best fit for the property.

3m x 4m Conservatory FAQ