Does a conservatory need foundations?

A conservatory built under 30 square meters and separate from the main house does not require building regulations. Therefore, technically, foundations are not a requirement.

Before you think it’s okay to go ahead without foundations, consider that any construction needs to be built on a solid, level, damp-proof footing to ensure stability and durability. We never recommend building a conservatory without foundations.

Many conservatories are now built with open access to the house or with integrated heating systems. This means they fall under the same bracket as an extension for permitted development rights and Building Regulations. Under Building Regulations, an extension does need the correct foundations.

How important are conservatory foundations?

Foundations for a conservatory are essential. Your conservatory structure will only be as strong as its foundations and without the correct preparation, groundwork and footings, it could start to subside.

If you’re constructing even a small lean-to conservatory, we would not recommend you build without making sufficient preparation of the ground and footings.

We prepare conservatory foundations for all of our conservatories and manage these as part of our full end to end project service.

Why do conservatories need foundations or footings?

A conservatory might look deceptively light because it’s made out of glass, but the weight of the glass needs considerable support to evenly transfer the weight into the ground.

Whatever type of soil you are building on is prone to shifting under different conditions. Under normal conditions, the ground swells and contracts on a constant basis. Throw in heavy rain and frost and this can significantly destabilise the ground.

Foundations apply a stable base to a structure to ensure that the ground isn’t constantly shifting and that the structure is safe to live in.

How deep should footings be for a conservatory?

There isn’t a specific minimum depth for conservatory foundations as they are dependent on the ground conditions and the size of the structure. A small uPVC conservatory will not require footing to be as deep as a large orangery style brick-based structure.

On average, standard conservatory foundations (with no obstructions) should be at least a metre (1,000mm) deep for a small lean-to, or 1500mm for a more sturdy structure - such as an orangery or large conservatory with brick walls. The width of the trench is variable.

For traditional extensions (that carry more weight), there is a general rule that foundations should be at least as deep as the distance from the front of the wall, to the edge of the foundation. But, no less than 1500mm deep.

Don’t forget, the depth of your foundations will dictate the maximum weight of your construction. This means that if you ever wanted to build a solid roof on your conservatory, or make your conservatory more solid with brick walls, your foundations could restrict you. It’s always better to future-proof your foundations during construction.

As a registered FENSA installer we calculate regulatory depth and width,  taking into consideration the load weight that the structure will place on the ground and other ground conditions, such as the proximity of trees, rivers and train lines.

Never start work on a conservatory without first speaking to your local authority about building regulations. Or, use a registered installer that will do this on your behalf.

Always get a second opinion if an installer tells you that you don’t need foundations at all.

How to prepare groundwork for conservatory

Can you build conservatory foundations over drains?

Drains and sewers are a problem for building work foundations and can dictate where you position your conservatory:

• You cannot build over a manhole with concrete foundations.
• You cannot build over or within 3 metres of a public drain/sewer without approval from the local water authority.
• You can possibly move a manhole to allow for building with approval from your local water authority.
• You can possibly build over a private drain or sewer linked to a single property.

If you do encounter public sewers, you must contact your local water authority before you start any work - even if your construction does not need planning permission or building regulations. You will have to pay for approval and for any relocation of manholes and drains.

If you have a private drain to a single dwelling, you can build over, but with special requirements. The foundations must be dug deeper than the bottom of the pipe to allow for the pipe to pass through a duct without concrete being poured directly on to the drain. You can only build over a maximum of six metres length of drain.

It’s advised to check the position of drains before you begin your project so you can fully understand the additional costs that will be incurred.

How to build a conservatory base

When constructing foundations, they are usually created by digging a trench in the ground to the required depth and then filling with concrete. In certain cases, steel reinforcement or piling rods will be required to strengthen the foundation. On top of the concrete, layers of hardcore, membrane and scree make up the underfloor of the conservatory.

• Concrete strip foundations A trench to the correct depth and width is dug and filled with concrete to the ground level and at least 150mm below the level of the damp-proof course.
• Piled foundations In special requirements, such as with a mixed soil layer (see above), or other unstable ground conditions, it’s necessary for extra measures to be taken. Steel rods are driven into the foundations before filling with concrete. The rods help to distribute the weight load of the structure into the ground.
• Conservatory base Hardcore is a solid material, like stone, that has been crushed and is laid on top of the concrete foundation to create a level base. At least 150mm of hardcore is recommended within your foundations. The compacted hardcore is then laid over with a bed of sand to provide an even surface.
• Damp-proofing A special layer of damp-proof membrane covers the base, to ensure that moisture from the ground does not rise up through the foundations.
• Floor insulation On top of the damp-proof membrane, a thick insulation layer is laid down to ensure that heat isn’t lost through the floor. We recommend using an insulation layer between 70mm and 100mm thick.
• Smooth screed floor A final layer of concrete is poured over the base and reinforced with steel, before applying the floor screed. Screed is made from cement, or a mixture of sand and cement, and is ‘smoothed’ into place using a levelling board over a raised timber bed.

This flat and smooth surface is now ready to construct the conservatory.

Can you underpin a conservatory?

Yes, you can underpin a conservatory, but you should consider if it’s cheaper to start again and build a new conservatory. If a conservatory has been built badly in the first place, there might be more problems than can be solved with underpinning.

Subsidence happens when the ground under your building shifts or drops.

For a conservatory, it might have been built without foundations at all, or the ground might have shifted. The major reason for subsidence in buildings are nearby trees.

As mentioned above, trees will suck the moisture from the ground causing contraction. Cutting a tree down can be just as problematic causing the ground to swell.

Before tackling underpinning, it might be possible to prune the tree, or to install a root barrier between the tree and the building. This would be substantially cheaper and less invasive.

Underpinning involves digging a trench around a structure and pouring concrete underneath to strengthen the foundations. The other option is to use steel piles driven into the ground with brackets that can support the base of the structure.

What is a no foundation conservatory?

Building a small conservatory without foundations might seem like a tempting way to save money, but can cause you a lifetime of problems:

• Ground not prepared properly and an uneven floor
• A shifting ground causing the conservatory to move and be unstable
• You might build over drains that you didn’t know where there (a huge problem if the sewers should ever flood)
• Water ingress from water-logged ground and heavy rain

Some companies offer to build conservatories without foundations, but if you want a quality durable structure, there are no shortcuts. We don’t ever recommend building a conservatory without foundations and foundations are always included as part of our service.

What do conservatory foundations cost?

Conservatory foundations can cost anything from several thousand pounds upwards depending on the complexity of the build. Footings can only be priced on an individual basis. Factors such as your type of soil, any drains or nearby trees can all significantly impact the cost of preparing your conservatory foundations.

We recommend that you always get proper advice from a trusted installer or registered builder before building your conservatory.

At Everest, we include foundations as part of our end to end conservatory design and installation service, so the worry and hassle is taken care of for you.

Read more about building regulations for foundations here.

LABC Homeowner’s guide to Conservatories

Information here is a guide only and not to be taken as legal advice. Updated July 2024.