Replacing a Conservatory With an Extension: What It Really Involves

The conservatory is unusable. Freezing from October to April, unbearable in July, and it's quietly become the place the bikes and the drying rack live. You want a proper room there instead.

We build home extensions across Northamptonshire and this is one of the most common projects we're asked about. Here's what replacing a conservatory with an extension actually involves: the planning position, the cost, the reason the old foundations almost never work, and how the build runs.

Can you replace a conservatory with an extension?

Yes. Planning rules treat conservatories and extensions identically, so a like-for-like replacement of a similar size usually falls within permitted development and needs no planning application. Building regulations are a different matter. The new extension must meet current standards for insulation, structure and drainage, which a conservatory was exempt from.

That distinction catches people out. The planning side is often the easy bit. The building regulations side is where the cost sits.

Do you need planning permission?

Often not. Under permitted development in England, a single storey rear extension can project up to 3 metres beyond the original rear wall on a terraced or semi-detached house, and 4 metres on a detached house. The Larger Home Extension route takes this to 6 and 8 metres respectively, subject to a neighbour consultation process.

Because your conservatory already sits within those limits (it had to, when it was built), replacing it with an extension of the same footprint is usually permitted development.

Things that change that:

• You want a bigger footprint than the conservatory. Fine, as long as you stay within the permitted development limits. Go beyond them and it's a planning application.

• The house is listed or in a conservation area. Permitted development rights are restricted or removed.

• An Article 4 direction applies. Some councils remove permitted development rights in specific areas.

• Previous extensions. Permitted development is measured against the original house as it stood in 1948 (or when first built, if later). If previous owners have already extended, you may have used up the allowance.

• The extension is more than 4 metres high, or more than 3 metres high within 2 metres of a boundary.

Even where you don't need planning permission, it's worth applying for a Lawful Development Certificate. It costs a fraction of a full application and gives you documented proof the work was lawful, which your buyer's solicitor will ask for when you sell. The Planning Portal sets out the full permitted development limits.

Why you can't just reuse the conservatory's foundations

This is the single biggest misunderstanding, and the reason "replacing" usually beats "converting".

Conservatory footings are typically 200 to 300mm deep. They were designed to carry a lightweight aluminium or uPVC frame and a polycarbonate roof. An extension needs foundations of at least 1 metre, and often 1.2 to 1.5 metres in the clay soils common across Northamptonshire, because it has to carry solid masonry walls and a tiled or warm flat roof.

The depth of foundations for a single storey extension is set by building control on site, based on soil, trees and nearby drains. Whatever number they give you, it will be several times what the conservatory sits on.

Underpinning existing conservatory footings is theoretically possible. In practice, the cost of underpinning usually meets or exceeds the cost of digging fresh foundations, and you're left with awkward junctions and a slab that may or may not be sound. Demolishing and starting again is almost always cleaner and often cheaper.

The same logic applies to the slab. Conservatory floor slabs are usually thin, uninsulated, and set at a level that suited a conservatory rather than a room that flows level with the house. If you want a continuous floor level from the kitchen into the new room (and you do), the slab is coming out.

How much does it cost to replace a conservatory with an extension?

Outside London, a single storey extension replacing a conservatory typically costs £1,800 to £3,000 per square metre for the shell. On a 15 square metre conservatory footprint, that's roughly £27,000 to £45,000, before fit-out.

The demolition itself is the cheap part. Stripping out a uPVC or aluminium conservatory usually costs £800 to £2,500, including skips and disposal, because the frame is bolted on rather than bonded into the wall. Removing the old slab and footings adds to that.

What drives the total:

• Foundations. Deeper is more expensive, and the depth isn't confirmed until the trench is open and building control has looked at it.

• Roof type. A warm flat roof with rooflights is cheaper than a pitched, tiled roof that ties into the existing roofline.

• Glazing. Bifolds, sliders and roof lanterns are where budgets run away. A run of premium aluminium bifolds can be £10,000 to £20,000 on its own.

• Opening up to the house. If you're removing the original rear wall to create an open plan space (and most people are), that means steels, and steels mean a structural engineer.

• Fit-out. Flooring, plastering, decoration, electrics, heating. And if it's becoming a kitchen, the kitchen itself is a separate budget entirely.

Add 10 to 15% contingency, plus professional fees for design, structural engineering, building control and any planning application.

What the new extension has to meet

A conservatory was exempt from most of the building regulations. An extension isn't. The new build has to comply with:

• Part L (energy efficiency). Insulated cavity walls, insulated floor, insulated roof, and glazing limited to around 25% of the floor area. The wall-to-wall glass of a conservatory won't pass.

• Part A (structure). Foundations, walls and roof designed to carry the loads, with structural calculations submitted.

• Part H (drainage). Any drain running under or near the extension has to be dealt with properly, with a build over agreement if it's a public sewer.

• Part K and Part N (glazing safety). Toughened or laminated glass in critical locations.

• Part P (electrical safety). New circuits designed, installed and certified by a competent person.

• Part F (ventilation). Trickle vents, extraction, and adequate air changes.

Building control inspects at each key stage: foundations, damp proof course, drainage, structure, insulation and completion. You get a completion certificate at the end, which you'll need when you sell.

The middle option: a warm roof

If a full replacement isn't in budget, the middle option is a warm roof. The polycarbonate or glass roof comes off and a solid, insulated, tiled roof goes on in its place.

This costs £4,000 to £8,000 for a typical conservatory and takes 3 to 5 days. It solves the temperature problem, which is the complaint most people actually have.

What it doesn't do is turn the conservatory into an extension. The space stays legally classed as a conservatory. The separating doors stay in place, the walls stay as they are, and a surveyor will still treat the floor area as non-habitable when valuing your house. It's a comfort upgrade, not a structural one.

We've covered the distinction between converting and replacing in more detail in our piece on whether you can turn a conservatory into an extension.

How the build runs

The sequence on a typical conservatory replacement:

1. Survey and design. Measured survey, design drawings, structural engineer's calculations. Two to eight weeks depending on complexity.

2. Approvals. Building regulations application, plus a Lawful Development Certificate or a full planning application if needed. If planning permission is required, it takes 8 weeks from validation.

3. Strip out. Conservatory dismantled, slab and footings broken out, site cleared. Usually under a week.

4. Groundworks. Trenches dug, inspected by building control, foundations poured, drainage laid, slab formed.

5. Shell. Cavity walls built, roof structure installed, extension made watertight.

6. Break through. The original rear wall comes out, steels installed, the new space opened to the house.

7. First fix and second fix. Electrics, plumbing, heating, plastering, flooring.

8. Completion. Final building control inspection and certificate.

Total time on site is usually 8 to 14 weeks for a straightforward single storey replacement. Add the design and approval time in front of that and you're looking at 4 to 6 months from first conversation to finished room.

Is it worth doing?

The financial case is usually straightforward. A poorly insulated conservatory is often valued at little or nothing as habitable floor space by a surveyor, and it can drag your EPC rating down. Replacing it converts dead area into space that counts.

Beyond the numbers, the honest reason people do it is that they get a room back. A conservatory you use two months a year becomes a kitchen-diner you live in every day. In our experience that's the thing homeowners actually mention afterwards, not the valuation.

Get a quote for a conservatory replacement in Northamptonshire

If you've got a conservatory you want replaced with a proper extension in Burton Latimer, or anywhere else in the county, speak to our team. We'll assess what's there, tell you honestly whether anything's worth keeping, and come back with a realistic figure and timeline. Finance is available through Phoenix Financial Consultants if you'd rather spread the cost.