Do You Need a Structural Engineer for a Conservatory? What the Exemption Does Not Cover

Conservatories occupy a strange position in the British home. They are the one bit of building work most people assume needs no real paperwork and no real design — you choose a style, somebody quotes, and a few weeks later there is a glass room where the patio used to be. Quite often that is genuinely how it goes. But the reason people ask whether they need a conservatory structural engineer is usually that somebody has mentioned the exemption, and exemptions are where the confusion starts.

The exemption is real. It just does not cover the part that matters most.

What the exemption actually covers

A conservatory can be exempt from most Building Regulations requirements if it meets a specific set of conditions. Broadly: it sits at ground level, it is under thirty square metres, it is separated from the house by external-quality doors or windows, more than half the wall area and three quarters of the roof are translucent, and the heating system is independently controlled.

Meet all of that and you avoid the thermal and energy requirements that would apply to an ordinary extension. That is the point of the exemption, and it is why conservatories can be built quickly and relatively cheaply.

Here is the part that catches people out. The exemption is about the conservatory. It says nothing about the hole you are about to make in the house it is being attached to.

The opening in the existing wall is not exempt

Almost every conservatory involves forming or widening an opening in an external wall — usually to fit patio doors where a window used to be.

That external wall is loadbearing. It is carrying the roof, possibly a floor, and its own weight, and all of that has to be picked up and carried across the new opening by a lintel or beam. Sizing that member is structural design work, it falls squarely under the Building Regulations, and it needs calculations regardless of whether the conservatory itself is exempt.

We have been called to houses where a wide opening was formed under a lintel that was never designed for the span, sometimes years earlier, and the first sign was cracking above the opening or a door that stopped closing properly. Neither is expensive to prevent. Both are tedious to put right afterwards.

Foundations, and why conservatory foundations are not automatically adequate

A conservatory is light, so the loads going into the ground are modest. That does not mean the foundation can be shallow.

Foundation depth in the UK is usually governed by what the ground does rather than what the building weighs. On shrinkable clay, seasonal moisture change causes the ground to move up and down, and a foundation founded above that zone moves with it. Nearby trees make it worse and push the required depth considerably further down.

There is a second issue specific to extensions of any kind. Where a light new foundation meets the heavier existing one, differential movement between the two is common. It shows up as cracking along the junction between old and new, which is why the connection detail matters as much as the depth.

Bungalows have their own complication

A good proportion of conservatories go onto bungalows, and bungalows are worth treating carefully.

With no upper floor, the roof structure is often tied by the ceiling joists alone. Form a wide opening in the wall below and you have removed some of what was restraining that wall at its base, on a building that has less inherent stiffness than a two-storey house to begin with.

It is entirely manageable. It simply needs someone to look at how the roof is restrained rather than assuming the opening only affects the wall it is cut into.

When a conservatory quietly stops being a conservatory

This one causes a surprising amount of trouble at sale.

The exemption depends on the conservatory staying thermally separated from the house. Remove the doors between the two, extend the central heating into it, or replace the glazed roof with a solid one, and it is no longer an exempt conservatory. It is an extension, and extensions require full Building Regulations approval.

Plenty of people do exactly this a few years after the conservatory goes up, without realising the status has changed. It surfaces during conveyancing when a buyer’s solicitor asks for a completion certificate that does not exist.

Replacing a glazed conservatory roof with a solid one is worth singling out, because it is popular and because it changes the loading substantially. The existing frame and the foundation were designed for a light roof. A tiled replacement can weigh several times as much, and whether the structure can take it is a calculation rather than a sales claim.

When you genuinely do need an engineer

• You are forming or widening an opening in an external wall. Nearly always the case, and nearly always needs a designed lintel or beam.
• The property is on clay, or there are mature trees nearby. Foundation depth becomes a real question rather than a standard detail.
• You are replacing a glazed conservatory roof with a solid one. The loading changes fundamentally.
• You are removing the separating doors or heating the space. It becomes an extension, with everything that follows.
• It is a bungalow with a wide opening. Worth checking how the roof is restrained.

And when you probably do not

If the conservatory goes onto an existing pair of patio doors with no change to the opening, sits on ground with no unusual features, keeps its glazed roof and stays separated from the house, then the structural question is genuinely small.

It is worth a conversation rather than a full appointment. We would far rather tell somebody they do not need us than be called in eighteen months later.

Frequently asked questions

Do I need Building Regulations approval for a conservatory?

Often not for the conservatory itself, if it meets the exemption conditions. But the opening formed in the existing wall is not exempt and does require approval, along with calculations for the lintel or beam.

Do I need planning permission?

Usually not, as most conservatories fall under permitted development. Size, height and position all matter, and the rules differ for listed buildings and conservation areas.

Can I put a tiled roof on my existing conservatory?

Sometimes, but it needs checking first. A solid roof weighs considerably more than glazing, and both the frame and the foundation were designed for the lighter load.

How deep do conservatory foundations need to be?

It depends on the ground rather than the building. On shrinkable clay, and particularly near trees, considerably deeper than people expect. A trial hole answers it directly.

What if my conservatory was built without approval?

It can usually be regularised retrospectively with a structural assessment to support the application. Worth sorting before you sell rather than during.

Thinking about a conservatory?

The useful moment to get structural input is while the opening is still a line on a drawing, because that is when it costs almost nothing to get right. A conservatory structural engineer can size the beam over the new opening, confirm what the foundation needs to do, and tell you plainly which parts of the job need approval and which genuinely do not.

We are a chartered structural engineering practice with offices in Peterborough and London, covering the East of England, Hertfordshire and Greater London, fully insured to £5 million professional indemnity.

Send us the plans or a photo of the wall you are opening up, and we will tell you what is involved.