Laying Conservatory Foundations

If you are considering building or adding a conservatory to your property then to ensure that it can withstand the elements and remain upright, it will need to be set on a suitable foundation.

In this project guide we look at how to build a conservatory foundation and everything involved.

This project will only apply to your conservatory if it is not governed by building regulations.

Do you Need a Foundation for Your Conservatory?

As with any form of construction, whether it’s a massive warehouse or a shed, to ensure that it remains standing upright through any form of weather and for as many years as possible, it will need to sit on a solid and stable foundation.

The job of a foundation is to transfer the weight of a structure down into the ground which then in turn keeps it stable and upright. Without it the structure would be unstable and suffer other issues such as damp and even subsidence.

Building Regulations and Conservatory Foundations

When it comes to conservatory foundations and the Building Regulations, the basic rule of thumb is that if your proposed conservatory is going to be at the same level as your home, involve the removal of external doors to access it and/or it is going to share the existing heating system, then it will be subject to the building regulations and thus have to be checked and signed off by Building Control.

This is due to the fact that it will need to meet minimum insulation and energy preservation standards and ensure that these are the same or better than the property it’s attached to.

With this in mind, a conservatory can be exempt from Building Regulations if it meets the following conditions:

• The conservatory is separated from the original structure by external doors, walls, or windows
• There are no drains within the conservatory
• The floor at ground level does not exceed 30 square metres
• A minimum of 75% of the roof is transparent e.g. made of polycarbonate sheeting or glass
• At least half the wall area is glazed
• Any heating in the conservatory is totally separate from the rest of the property

When it comes to foundations and Building Regulations, your Building Control officer/office will likely state that the foundations will need to be at least 1m deep for smaller structures, but if larger, then more than likely 1.5m deep.

More information on this can be found in our foundations project.

Conservatory Foundations and Planning Permission

If you are unfamiliar with either planning permission or Building Regulations then it’s important to state that these are 2 entirely different entities. Even though you may not need planning permission, you may still need Building Regs approval.

Under certain conditions, you can construct a conservatory under permitted development without the need for planning permission, but as said, this can depend on many things.

To know exactly whether you need planning permission for your conservatory, your best option is to contact your Local Authority Building Control office and inform them of your plans and they will then be able to advise on whether you will or won’t.

When it comes to not needing planning permission, the conditions are summarised as follows:

• Is at the rear of your home and not more than 4m in height
• When on the side of your home, is not more than 4m high and is not wider than the original property
• Features eaves that are lower than the highest part of the property
• Conservatory eaves are no higher than 3m if within 2m of the property boundary
• Is no more than a 6m extension from the rear of the property
• Covers no more than 50% of the footprint of the original property

How Deep do Conservatory Foundations Need to be?

Building a conservatory is a fantastic way to add extra space and value to your home, but getting the foundations right is essential.

In the UK, the depth of conservatory foundations depends on several factors, including soil type, nearby trees, and the weight of the structure.

Properly laid foundations ensure your conservatory is stable and prevents issues like cracking or sinking over time.

In most situations, e.g. for a small lean-to style conservatory, foundations will need to be around 1000mm deep.

However, for larger, more substantial conservatories such as those with brick walls or large, heavy timbers such as some orangeries, foundations may need to be 1500mm deep.

The depth of any foundations will also determine what future work can be carried out. For example, if in years to come, you decide to put a solid roof on your conservatory, the additional weight may mean that your foundations will no longer provide adequate support as they are not thick enough.

So, with this in mind, it’s always a good idea to build with the future in mind.

Ground Types and Conditions

When building a conservatory, the type of ground beneath your property plays a crucial role in determining the depth and structure of the foundations.

Different ground types vary in stability, moisture retention, and strength, all of which affect how your conservatory’s foundations are designed.

Understanding these factors ensures a safe, long-lasting structure that won’t shift, settle or slump over time.

• Clay Soil: Clay is common in many parts of the UK and is known for expanding when wet and shrinking when dry. This movement can cause instability, so foundations often need to be deeper (up to 1,500mm) to reach more stable subsoil layers.
• Sandy Soil: Sandy soil drains well and is less prone to movement, but it can lack strength. For conservatories built on sandy ground, reinforced foundations may be needed to spread the load evenly.
• Chalk Soil: Chalk is generally stable and provides a solid base for foundations. Standard foundation depths of around 600mm to 1,000mm are usually sufficient for conservatories on chalky ground.
• Loam Soil: A mix of sand, silt, and clay, loam is a balanced and stable ground type, ideal for construction. Foundation depth may vary but is typically between 600mm and 1,000mm.
• Peat Soil: Peat is soft, organic, and prone to significant movement, making it one of the least stable ground types. Foundations may need to be much deeper or reinforced with piles to provide stability.
• Nearby Trees: If there are trees nearby then they can cause soil movement, particularly with clay soils, so when planning a conservatory build take note of the size and type of trees nearby and their likely root structure.
• Drains: There are very strict rules when it comes to drains and you will need to notify your local authority if there are any in or around your proposed build site. You cannot concrete over a manhole and you cannot build within 3m of a public drain/sewer without authority approval. Drains and manholes can be moved, however this is often extremely expensive.

Before a spade or digger bucket touches the ground, it’s essential to identify the soil type. This can be done through a simple soil survey, often carried out by builders or structural engineers.

These professionals can assess factors like soil composition, water content, and load-bearing capacity, providing guidance on the right foundation depth and design.

Types of Conservatory Foundations

As we have mentioned above, using the right type of foundation for the size of your conservatory and the ground you’re building on is imperative.

With this in mind, there are several different types of foundation design to select, depending on the conditions you’re dealing with:

Strip Foundations

Strip foundations are one of the most common types used for conservatories. This method involves digging a trench along the perimeter of the conservatory, filling it with concrete to create a stable base.

Strip foundations are ideal for smaller conservatories built on stable ground, such as chalk or loam soil.

They’re cost-effective and relatively simple to install, making them a popular choice for standard builds.

Raft Foundations

Raft foundations are generally used for more specialist builds where soil conditions are less than ideal and involve creating a single, large concrete slab that spreads the weight of the conservatory across a wide area.

As said, this type of foundation is particularly useful for sites with weak or uneven ground, such as peat or sandy soils as by distributing the load evenly, raft foundations help prevent sinking or shifting.

They’re also a great option for modern conservatories with heavier materials like brick or tiled roofs.

Pile Foundations

In areas with highly unstable ground or close to large trees, pile foundations may be necessary.

These foundations use deep concrete or steel posts (piles) driven into the ground to reach stable soil layers.

Pile foundations are more complex and expensive than other types, but they provide excellent stability and are often used for larger conservatories or challenging soil conditions.

Pad Foundations

Pad foundations are smaller, isolated concrete bases designed to support specific load-bearing points, such as conservatory columns.

These are typically used for lightweight conservatories or as part of a hybrid system with other foundation types.

Pad foundations work well on firm ground but may not be suitable for clay or highly compressible soils.

Concrete Base Foundations

Concrete base foundations combine elements of strip and raft systems, providing a solid concrete floor that acts as both the foundation and the base for your conservatory flooring.

This is a great option for conservatories that will double as permanent living spaces, offering excellent stability and insulation. However, it can be more expensive and time-consuming to install.

How to Build a Conservatory Foundation

Once you have determined the type of soil you’re dealing with and your builder, engineer, building control office has specified the type and size of foundation needed for your conservatory build, it’s now time to get it constructed.

Even if you are not subject to Building Regulations, it’s a great idea to follow them anyway.

For your conservatory to stay upright for any length of time it makes sense to follow the guidelines regardless as they are there for your benefit, not the councils.

Step 1: Plan and Measure

The very first job is to mark out your foundation area for your conservatory using stakes and string.

Ensure the dimensions are accurate and match your plans and also check for underground utilities like pipes or cables, which could affect where you dig, however this should all be done prior to you starting in the planning and ground survey stage.

At this point, as we have said, it’s also essential to assess the soil type and ground conditions. Different soils, such as clay or sandy soil, will require specific foundation depths or designs.

The area you are building on should not be wet or spongy, so dig down to what you believe is solid ground, cut a square peg, 50mm x 50mm and bang it into the bottom of the trench with a lump or club hammer.

You should not be able to drive the peg in any more than 150mm (6inches) without difficulty. If this is the case then the ground is suitable enough to work with.

Step 2: Excavate the Foundation Trench

Once all marked out, the next task is to then dig out your foundation trench.

Using a mini digger or manual tools such as a spade, pick etc. if space does not allow for plant machinery, excavate a trench around the perimeter of the conservatory site, following your lines.

For most UK soil conditions, the trench should be at least 1,000mm deep, but this can vary depending on the site and nearby factors, like trees. The exact depth should be stated by your Building Control officer.

The width of the trench will depend on your structure’s weight, typically around 300mm to 600mm. It should be wide enough to accommodate whatever thickness of wall you have in the centre, with a 100mm overhang either side.

For example, If you have an internal skin of blockwork of 100mm, plus a 50mm cavity, plus an external skin of 100mm brickwork this is a wall thickness of 250mm. Add 100mm either side of this and the trench width should be 450mm. Again this calculation does not supersede any given with building regulation requirements.

Make sure the base of the trench is level and free from loose soil or debris and any rocks. A smooth base will ensure it’s structurally sound and won’t have any weak sections.

Step 3: Prepare the Base

Once the trench is dug out to the correct depth, add a layer of hardcore or compacted gravel of the correct size to create a stable base. It should be at least 150mm thick. Again, if you are unsure on what size stone to use, consult with your local Building Control office.

This layer will help with drainage and will provide extra support for the concrete above it.

Compact the hardcore using a tamper or vibrating plate to ensure it’s evenly distributed and firm, without any gaps that could collapse and case weak spots.

Step 4: Pour the Concrete

Once the trench and base is all ready to go, the next and most important job is the concrete pour.

Mix concrete to the appropriate ratio (1 part cement, 6 parts mixed aggregate (ballast e.g. 2 parts sand, 4 parts gravel) and pour it into the trench – see our mixing concrete project for help if needed. Again, if you have a large area to do, try to buy in some pre-mixed, it will save you a huge amount of time and effort on mixing.

Fill the trench to the desired level, ensuring the concrete is smooth and level. Use a spirit level to check accuracy as you work.

Allow the concrete to cure for at least 48 hours before proceeding with construction. Curing time can vary depending on weather conditions and temperature, so adjust as needed.

Step 5: Build the Foundation Walls

The final stage of the core foundation work is now to construct the dwarf walls to form the base of the external walls of your conservatory (if you are using this method). This is the normal process unless you are building straight off of the base.

Using bricks or concrete blocks, build the foundation walls to the required height. Normally blocks for the inner skin and then bricks for the outer to give it a little more of an aesthetic finish. These walls will support the conservatory frame and ensure stability.

Step 6: Building the Conservatory Base

Although technically this is part of the whole process it is in some way slightly different as the above steps deal with the supporting walls and footings whereas the following steps deal with the base that essentially forms the floor area.

Again, this should start with a flat, smooth base, with a compacted level of hardcore at least 150mm thick.

Next should come a 25mm layer of sand or binding as it’s known. This layer creates a cushion for the DPC or damp proof membrane, to prevent any punctures in this.

After the binding layer will now come the damp proof membrane to prevent any damp rising up into the interior.

After the DPC membrane you should then add a 75mm layer of insulation. This is normally in the form of solid insulation sheeting such as Kingspan, Celotex or the similar. This simply lays across the top of the membrane.

After the insulation layer comes the concrete slab that forms that actual interior base of the conservatory. This will need to be 100mm thick and again is simply poured over the insulation, levelled and smoothed and then left to cure.

Once the concrete pad has cured, the final finish is then to form a 50mm sand/cement screed. This is the finishing layer so needs to be absolutely flat and smooth. For detailed information, see our project on how to lay a screed.

Once completed your conservatory base should look similar to this:

Laying a conservatory foundation, which includes the footings and the base, may appear to be a massive, in-depth job if you have never attempted anything like this before, however if you know the process and take your time, this is something that can certainly be done on a DIY basis.