The short answer
Most conservatories are built on a concrete raft or a dwarf wall on a strip foundation — the right choice depends on the soil conditions, slope and drainage at your site. Poor ground, trees nearby or sloping gardens can add significantly to foundation costs. See how long to build a conservatory for how the base fits into the project timeline and conservatory cost for how groundworks affect the total price.
The base and foundations are often the least-discussed part of a conservatory project, but they are fundamental to its long-term performance. A conservatory frame that settles or shifts on a poorly specified base can lead to cracked glass, leaking seals, sticking doors and structural problems. The good news is that for most gardens with straightforward ground conditions, a concrete raft is a well-understood and reliable solution that an experienced installer or groundwork contractor can lay in one or two days. The complications arise with sloping gardens, clay soils, trees, or areas with underground drainage.
Conservatory base at a glance
- Concrete raft Most common — suits flat, good-ground sites
- Strip foundation + dwarf wall Suited to sloping gardens or softer ground
- Piled foundations Required where ground conditions are poor
- Minimum depth Typically 150–300 mm for a concrete raft
- Proximity to trees Tree roots can cause heave — specialist advice needed
- Drainage Must divert or protect any existing drains
The concrete raft foundation
A concrete raft is the most common and straightforward foundation for a conservatory. The topsoil is excavated to a depth of typically 150–300 mm (more on softer or clay soils), hardcore is laid and compacted, and then concrete is poured over a steel mesh reinforcement. The raft provides a flat, rigid platform that supports the frame uniformly. For a simple, flat site in good-draining soil, a raft typically costs £1,500–£3,000 depending on size and ground conditions. It is the most cost-effective base where it is appropriate, and most conservatory installers have their own groundwork team or a preferred subcontractor to handle it.
The concrete must cure for 7–14 days before the frame is erected. During that time the raft should be protected from frost and from loading. On a good site, the process is reliable and repeatable; the complications come when the site departs from the simple flat-on-good-ground ideal.
Strip foundation and dwarf wall
Where the garden slopes, or where the conservatory is to be raised slightly above the house floor level, a strip foundation with a dwarf wall (a low masonry wall on a strip concrete foundation) is a common solution. The strip runs around the perimeter of the conservatory, and the dwarf wall creates the level platform for the frame to sit on. This approach is more expensive than a simple raft because it involves more excavation, masonry work and a longer programme, but it is often the right answer for sloping gardens and traditional brick-based conservatories. An orangery will almost always be built on a dwarf wall base as part of its masonry construction.
| Foundation type | Best for | Typical cost |
|---|---|---|
| Concrete raft | Flat site, good ground | £1,500–£3,500 |
| Strip + dwarf wall | Sloping ground, heavier frame | £3,000–£7,000+ |
| Piled | Poor ground, clay, trees | £5,000–£15,000+ |
| Suspended timber floor | Steep slopes, access constraints | £4,000–£10,000+ |
Trees and roots — a significant complication
If there are trees within the footprint or close to the proposed conservatory site — particularly large trees or species known to cause heave (oak, beech, poplar, willow) — the foundation specification changes significantly. Tree roots can cause both heave (swelling in clay soils) and settlement (shrinkage as roots draw moisture). A structural engineer should advise on foundation depth and specification where trees are involved; pile foundations extending below the root influence zone may be required. This is not a situation to leave to the conservatory installer’s judgement alone — independent structural advice is worthwhile. Note also that felling or significant pruning of certain trees may require consent (trees in conservation areas or subject to a Tree Preservation Order).
Drainage from the conservatory roof
The conservatory roof will collect significant rainfall that needs to be directed safely away. A roof drainage system — guttering and downpipes — must be connected to a suitable outlet: an existing soakaway, a surface water drain or a rainwater butt. Never connect conservatory roof drainage to the foul drain (the sewerage system), as this is both illegal and can cause sewage surcharge in heavy rain. The drainage design should be agreed with the installer at survey stage and confirmed in the specification. This page provides general guidance only — foundation design for conservatories should be confirmed by a qualified installer and, where ground conditions are complex, by a structural engineer.
Get your conservatory project assessed
A reputable installer will assess your site conditions and include the base specification in their written quote. Compare quotes that include groundworks, not just the frame price.
Frequently asked questions
What type of base does a conservatory need?
Most conservatories are built on a concrete raft for flat sites with good ground. Sloping gardens or softer ground may need a strip foundation with a dwarf wall. Poor ground conditions, clay or trees nearby may need piled foundations.
How deep does a conservatory foundation need to be?
A concrete raft is typically 150–300 mm deep on good ground. Strip foundations on clay or near trees may need to be much deeper — a structural engineer should advise if ground conditions are uncertain.
Can a conservatory be built on an existing patio?
Sometimes, if the patio is in good condition, level and of sufficient depth, it can form part of the base. But an existing patio rarely meets the specification for a conservatory foundation on its own — the installer should assess and confirm.
What happens if there are drains under the conservatory area?
Existing drains must be identified, diverted or bridged before the base is laid. Building over an unprotected drain can cause structural problems and is not permitted over public sewers without water company approval.
Sources & further reading
- LABC — Local Authority Building Control guidance on conservatory foundations, drainage and structural compliance
- GOV.UK — Building Regulations Part A (structure) and Part H (drainage) as applied to conservatory bases
- Glass & Glazing Federation — conservatory groundworks standards and installation best practice
- Planning Portal — guidance on trees, drainage and site conditions affecting conservatory projects
This is general information about conservatories and orangeries in the UK, not planning, structural, legal or financial advice. Costs are typical illustrations only and are not quotes for any specific project; actual prices vary with size, site conditions and your chosen installer.