Converting a Conservatory to an Extension: UK Costs and Rules for 2026

What does converting a conservatory actually involve?

Conversions sit on a ladder, and the price climbs with each rung:

• Solid roof replacement — the glazed or polycarbonate roof comes off and a lightweight insulated tiled roof goes on. The fastest, cheapest route; price one for your size with our conservatory roof cost calculator.
• Roof plus thermal upgrade — new roof, plus insulated plasterboard to the dwarf walls, upgraded glazing and a properly insulated floor.
• Structural conversion — some or all of the frames are replaced with insulated cavity walls, often with the foundations strengthened, so the room is an extension in all but history.
• Knock-through — at any rung, removing the doors between house and conservatory to create one open space. This is the step that formally ends the building-regs exemption, so it is usually done alongside the thermal work, not before it.

Which rung you need depends almost entirely on what the original installer left you — which is why every good conversion starts with a structural survey of the base.

How much does the conversion cost in 2026?

The honest answer is £15,000–£40,000 for a full habitable conversion, with roof-only projects starting lower. Where your project lands depends on three things:

• The base. If the original foundations can carry a solid roof, you save thousands. If they need underpinning or rebuilding, you are closer to new-extension money.
• How much frame survives. Keeping sound dwarf walls and some glazing keeps costs down; replacing frames with cavity walls is where budgets grow.
• The knock-through. Removing the rear wall doors often needs a steel beam and structural calculations.

Allow professional fees of 10–15% where an engineer or architect is involved, plus a 10–15% contingency — conversions uncover more surprises than new builds. Builder quotes to homeowners normally include VAT. Compare the totals against a fresh build using our single-storey extension cost guide.

Conservatory conversion costs (2026)

Typical 2026 prices for converting an existing conservatory of around 12–16m², from a roof swap to a full rebuild. Every project rests on what the original base can carry, so treat these as planning figures until a builder has surveyed yours.

Why the building-regs exemption disappears

A conservatory is only exempt from building regulations while it meets every one of these conditions: it is under 30m², thermally separated from the house by external-quality doors, glazed to the rules, and heated independently on its own controls. A conversion deliberately removes those conditions — that is the whole point of the project.

So once you fit a solid insulated roof, open the room to the house or plumb it into the central heating, the space is treated as habitable and the work must comply:

• Thermal performance — roof, walls, floor and glazing all have U-value targets to hit
• Structure — the base and frames must be proven capable of carrying the new roof
• Ventilation, drainage and electrics are inspected like any extension

Doing this properly matters at resale: a converted conservatory without a completion certificate is a classic surveyor red flag that can stall a sale or knock money off an offer.

Planning permission for conservatory conversions

Planning is usually the easy half. If you are converting within the existing footprint — same walls, same roofline or lower — most projects need no planning application at all, because the volume already exists. Where the conversion enlarges the structure, normal permitted development rules for rear single-storey extensions apply: 3m deep for semis and terraces, 4m for detached homes (6m and 8m via prior approval), maximum height 4m, and eaves no higher than 3m within 2m of a boundary.

Watch the exceptions:

• Conservation areas and listed buildings — PD rights are void, so even a like-for-like roof swap deserves a call to the council
• Removed PD rights — common on newer estates, buried in the original planning conditions
• Original planning conditions on the conservatory itself — occasionally a conservatory was approved on condition it stays a conservatory

When in doubt, check the Planning Portal or ask the council for a lawful development certificate — useful paperwork when you sell.

Convert or knock down and build new?

The alternative every conversion should be tested against is demolition and a new single-storey extension at £1,900–£2,600 per m². For a 15m² footprint that means roughly £28,500–£39,000 — overlapping the top of the conversion range.

• Convert when the base is sound and the shape works: you keep thousands of pounds of existing structure and finish weeks sooner.
• Rebuild when the foundations fail a survey, the footprint is wrong for how you live, or you want a design the old frame cannot deliver. Paying twice — a compromised conversion now, a rebuild in five years — is the worst outcome.

Get both options priced. Our house extension cost guide covers new-build numbers in detail, and the conservatory extension guide shows what a hybrid new build looks like if you start again.

FAQs: Converting a Conservatory to an Extension