A conservatory base is everything below the frames: the foundations dug into the ground, the floor slab, and usually a low dwarf wall of brick or block that the windows sit on. It carries the whole structure and keeps damp out, so it decides whether the conservatory stays level, dry and crack-free for decades. Most conservatories are exempt from Building Regulations, which means nobody from building control inspects the base, so getting it right is down to the installer. This page explains how a base is built, the damp-proofing, drains and manholes, and the clay soils and trees that matter around Windsor and the Thames Valley. For styles and roofs, see our main conservatories page.
The parts of a conservatory base
- Foundations. Concrete strip or trench-fill foundations under the walls, taken down to firm ground. Depth depends on the soil and nearby trees.
- Dwarf walls. Low walls, typically built as an insulated cavity wall with an outer skin chosen to match the house brick. Some designs use full-height glazing instead, with a low base course.
- Floor. Compacted hardcore, sand blinding, a damp-proof membrane, insulation, then a concrete slab or screed ready for the floor finish.
- Damp-proof course (DPC). A barrier in the walls that links with the floor membrane, so moisture cannot rise into the room.
The exemption and what it means for your base
The Planning Portal says a conservatory is normally exempt from Building Regulations when it is built at ground level, is under 30 square metres, is single storey, is separated from the house by external-quality walls, doors or windows, and has an independent heating system with separate controls. Its glazing and fixed electrics must still comply. Exemption does not mean the base can be skimped; it means the installer carries the responsibility. If the room is not exempt, for example because it has a solid roof, is over 30 square metres or opens into the house, building control will check foundations, floor insulation and damp-proofing. LABC’s solid roof guidance notes that a 150mm concrete strip or reinforced slab on original ground is usually adequate for a lightweight roof replacement, but that foundations over drains, near trees or on filled ground need extra thought.
Damp-proofing
Government guidance on Part C of the Building Regulations says new extension walls need a damp-proof course at least 150mm above ground level. In practice, the most common fault we see on older conservatories is a patio, path or flowerbed that has since been raised above the DPC, letting rain splash and soil bridge the barrier. We set floor and DPC levels to suit the house and the garden, and leave a clear zone around the base.
Where the house has a suspended timber floor, the airbricks that ventilate it must not be blocked by the new base. They are normally extended with ducts through to the outside.
Drains, gullies and manholes
The back wall of a house is where drains, gullies, rainwater downpipes and manholes tend to sit, which is exactly where a conservatory goes.
- Public sewers. Thames Water says you will likely need a build-over agreement if you build within 3 metres of a public sewer or within 1 metre of a public lateral drain, and that you should not start work before getting one. Planning permission does not give permission to build over a sewer.
- Shared drains. Many drains that were once private are now public. As Norton Rose Fulbright explains, sewers connected before 1 July 2011 transferred to the sewerage companies on 1 October 2011, so a shared run under neighbouring gardens may now need water company consent.
- Manholes and gullies. Inside the footprint, these are usually moved outside the base, or covered with a sealed access cover if relocation is not practical and the drain owner agrees.
- Downpipes. The house downpipes may need rerouting around or through the conservatory roof, with new gullies outside.
Thames Water suggests a drainage survey or its sewer maps if you are unsure where pipes run; we look for covers and gullies at survey and allow for them in the plan.
Clay soil and trees around Windsor and the Thames Valley
Ground conditions vary sharply in this area. The British Geological Survey’s notes for the Windsor and Bracknell map sheet describe a mix of London Clay, river terrace gravels, alluvium on the floodplain and, under Windsor Castle itself, an outcrop of chalk. The BGS notes that clays in the London Clay and Reading formations may undergo significant volume change as their moisture content varies, that drying clays can crack and swell, damaging buildings, and that floodplain alluvium can be variable with low bearing capacity. The BGS also maintains a shrink-swell hazard dataset for London and the Thames Valley, listing tree growth and removal among the causes of ground moisture change.
That matters for a conservatory because trees draw water from clay, shrinking it in dry summers, and removing a large tree can let the ground swell. NHBC Standards for foundations in shrinkable soils set minimum depths of 0.75m, 0.9m and 1.0m for low, medium and high shrinkability soils outside tree influence, with deeper foundations within the influence of trees and precautions against heave. NHBC also states that root barriers are not an acceptable alternative. Your conservatory is not an NHBC new home, but these figures are a sensible benchmark, and we dig trial holes where the soil or nearby trees suggest it.
If a tree near the proposed conservatory is protected by a Tree Preservation Order you need council consent for work on it, and in a conservation area you must give six weeks’ notice, according to GOV.UK guidance. Check before anyone prunes roots or branches.
Dwarf walls or full-height glass?
Dwarf walls give you somewhere to place furniture and radiators, hide clutter and help the room feel more like part of the house. Matching brick ties the conservatory to the home. Full-height glazing gives more light and a better garden view, and suits contemporary designs with bifold doors or patio doors. Either way, the base must be level and true so the frames sit square.
Floors and thresholds
Most people want the conservatory floor level with the house so there is no step at the doorway. Floor insulation under the slab makes a noticeable difference to comfort. Underfloor heating is possible, but it must be independent of the house system, with its own controls, to keep the exemption.
Reusing an existing base
If you are replacing an old conservatory, the existing base may be reusable. We check for cracks, movement, damp, a bridged DPC and whether the foundations will carry the new design, particularly if you are moving to a heavier glass or solid roof. See replacement conservatory roofs for when a new roof on the old base makes sense.
Common base mistakes
- Shallow foundations on clay close to trees.
- Building over a manhole or public drain without consent.
- Patio levels later raised above the DPC.
- Blocked airbricks under the house.
- No floor insulation, leaving cold feet all winter.
Start with a proper survey
We look at the ground, drains, trees and house wall before designing anything, then build the base with our own team. Call 01753 965 005 or request a free quote. We cover Windsor, Maidenhead, Berkshire, Buckinghamshire, Surrey, west Kent and north West Sussex. You may also like our pages on T-shaped conservatories and orangeries.
Frequently asked questions
How deep do conservatory foundations need to be?
It depends on the soil and nearby trees. NHBC Standards use minimum depths of 0.75m, 0.9m and 1.0m for low, medium and high shrinkability clay outside tree influence, with deeper foundations near trees. Firm, non-clay ground may need less. We judge depth at survey and dig trial holes where clay or trees suggest it.
Can I build a conservatory over a manhole or drain?
Sometimes, but it needs planning. Thames Water says you will likely need a build-over agreement within 3 metres of a public sewer or 1 metre of a public lateral drain, and that work should not start before you have one. Manholes inside the footprint are usually moved outside or fitted with a sealed access cover.
Does a conservatory base need Building Regulations approval?
Not if the conservatory is exempt: ground level, under 30 square metres, single storey, separated from the house by external doors or windows and independently heated. Building control does not then inspect the base, so quality depends on the installer. A solid roof, a larger room or a knock-through into the house means approval is required.
Is clay soil a problem for conservatories near Windsor?
It can be. The British Geological Survey notes that London Clay in the Windsor area may change volume significantly as its moisture content varies, and trees speed that up by drawing water out in dry summers. That can crack walls and floors if foundations are too shallow, so we check the soil and nearby trees before setting the foundation depth.
How high should the damp-proof course be on a conservatory?
Government guidance on Part C says new extension walls need a damp-proof course at least 150mm above ground level. The DPC must link with the floor membrane so moisture cannot rise into the room. Raising a patio or flowerbed against the base later can bridge the DPC, so keep that clearance in mind when landscaping.