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Foundations, DPC, slab and dwarf wall — the bit that decides whether a conservatory lasts 10 or 30 years.
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Top tips
Hiring a conservatories, without the regret.
Four moves that separate a smooth job from a nightmare.
Foundations too shallow
300mm "rule of thumb" doesn't work in UK clay soils which can heave 200mm seasonally. 600mm minimum, 1m for clay.
Slab not tied to perimeter
A "floating" slab can settle differently from the brick walls. Mesh reinforcement should tie into the perimeter beam.
DPC tie-in missing
Where the new conservatory meets the existing house, the DPC must be continuous. Cheap builds leave a damp path.
Insulation missed
100mm PIR under the slab is standard for habitable use. Cheap builds skip it and the floor is permanently cold.
Costs & timeline
Know what it costs. Know when it ends.
Indicative UK ranges and a typical week-by-week schedule.
Cost range
By job type
Inc. VAT · 2026
Source: NMT quotes
Source: NMT quotes
Excavation & disposal (mini-digger)1 day
£250–£800
Trench foundations (600mm dep)Concrete, perimeter
£800–£2k
Trench foundations (1m, clay soil)Deeper, extra concrete
£1.5k–£3.5k
Brick dwarf wall (cavity)To cill height
£800–£2.5k
Insulated reinforced slab150mm concrete + insulation
£800–£2.5k
DPC tie-in to existing houseCritical detail
£200–£600
!
Quote spread is typically ± 18% — always get 3 quotes.
Timeline
Mid-size conservatory · week by week
Typical
5 phases · 5 wk
5 phases · 5 wk
W1
W2
W3
W4
W5
Foundations + dwarf wall
Frame + glazing
Roof + ridge
Electrics + flooring
Snags + handover
!
Schedule slips on dependencies — pad each phase by 10–20% for real-world delays.
At a glance
Infographic · VideoThe Conservatories briefing.
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Conservatories quick-view
Video guide
Conservatories explained
The base and foundations of a conservatory are the make-or-break element — cheap foundations crack the brickwork above within 2 years. Expect to pay £1,800–£7,500 for a properly built UK conservatory base depending on size and soil conditions.
What Does the Work Invovle?
The builder excavates a trench around the perimeter (600mm minimum depth, 1m for clay or made-up ground) and a slab area to subbase level. Trenches are filled with C20 or C25 concrete, allowed to cure 7 days. Brick dwarf walls (cavity construction, 100mm cavity, insulated) are built up to cill height with a DPC at the right level. The slab is poured: 150mm consolidated hardcore, sand blinding, 1200-gauge DPM (lapping over the perimeter DPC), 100mm PIR insulation, 100mm reinforced concrete with A142 mesh. Underfloor heating pipes are usually tied to the mesh.
Typical Costs
| Item | Low (£) | High (£) | Notes |
|---|---|---|---|
| Excavation & disposal (mini-digger) | 250 | 800 | 1 day |
| Trench foundations (600mm dep) | 800 | 2,000 | Concrete, perimeter |
| Trench foundations (1m, clay soil) | 1,500 | 3,500 | Deeper, extra concrete |
| Brick dwarf wall (cavity) | 800 | 2,500 | To cill height |
| Insulated reinforced slab | 800 | 2,500 | 150mm concrete + insulation |
| DPC tie-in to existing house | 200 | 600 | Critical detail |
| Underfloor heating ready | 300 | 1,200 | Loops in slab |
How Long Does It Take?
1–2 weeks total. Excavation and concrete pour take 3–4 days; concrete cure 7 days; brick dwarf wall 3–5 days; slab pour and cure 5 days. Frame can start when slab is cured.
DIY or Professional?
Professional groundworker or builder. Foundation depth and concrete spec are critical — get them wrong and the brickwork cracks. Building inspectors don't sign off bad foundations.
Choosing the Right Tradesperson
A general builder with extension experience, or a specialist groundworker. The conservatory installer often sub-contracts the base build. Insist on written confirmation of foundation depth, concrete grade, and DPC tie-in detail.
UK Regulations
Foundations must follow Approved Document Part A (structure). For exempt conservatories the building inspector doesn't sign off, but the foundations still need to be structurally adequate. For Building-Regs-notifiable orangeries and large conservatories, full inspection at the strip foundations and DPC stages.
Common Problems
- Foundations too shallow: 300mm "rule of thumb" doesn't work in UK clay soils which can heave 200mm seasonally. 600mm minimum, 1m for clay.
- Slab not tied to perimeter: A "floating" slab can settle differently from the brick walls. Mesh reinforcement should tie into the perimeter beam.
- DPC tie-in missing: Where the new conservatory meets the existing house, the DPC must be continuous. Cheap builds leave a damp path.
- Insulation missed: 100mm PIR under the slab is standard for habitable use. Cheap builds skip it and the floor is permanently cold.
Frequently Asked Questions
How deep should conservatory foundations be?
600mm minimum into firm subsoil. 1m in clay or made-up ground. Soil tests at the site determine the exact depth required.
What concrete strength do I need?
C20 or C25 for foundations. C25 minimum for the slab. Pre-mixed delivered by Cemex, Hanson, etc — site-mixed is cheaper for small jobs but inconsistent.
Why does the slab need insulation?
For thermal comfort and Part L compliance. 100mm PIR under the slab gives a warm floor and saves on heating.
Can I build a conservatory on a paved patio?
No — paving is not a foundation. Excavate down to firm subsoil and pour proper concrete trenches.
What if my soil is clay?
Clay heaves seasonally — foundations need to go 1m+ depth to be below the active layer. Some soils need pile foundations.
How long after the foundation can I build?
7 days cure for concrete foundations, 5–7 for slabs. Total foundation-to-frame is typically 2 weeks.
This guide was written with AI assistance and is intended for general information only. Prices are estimates based on UK averages and may vary by region. Always get at least three quotes and consult a qualified professional before starting any work.
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