Foundations are the part of a conservatory nobody sees, yet they are the part that decides whether it stays dry, level and crack free for decades. Most homeowners are surprised by how much the base can vary in price, and by how much depends on what is under the lawn. Here is an honest look at what you should expect to pay around Bristol and why.
For a standard lean-to or Victorian conservatory of around 3m x 3m to 4m x 4m, a full base with strip or trench fill foundations, a concrete slab, dwarf walls and damp proofing usually lands somewhere between £2,500 and £6,000 in the Bristol area. That includes excavation, hardcore, concrete, blockwork below the damp proof course and disposal of spoil.
Larger or more complicated shapes, such as P shaped or gable fronted designs over 20 square metres, tend to run from £6,000 to £10,000 or more. As a rough rule, budget between £150 and £250 per square metre for a conventional base, before the frames and glazing are even considered.
A dwarf wall base sits at the upper end because of the extra brickwork and the need to match your existing facing brick. A full height glass to floor design has less masonry but still needs the same depth of footing, so the saving is smaller than people expect.
Ground conditions matter more than anything else. Much of Bristol sits on clay, particularly across the northern suburbs and out towards Keynsham and Yate, and clay shrinks and swells with the seasons. Where trees are nearby, footings often need to go down 1m to 1.5m rather than the usual 600mm to 750mm, which adds concrete, labour and muck away. Made up ground, old garden walls and buried rubble are common in older parts of the city such as Bedminster, Easton and Bishopston, and they can mean digging deeper to find something solid.
Access is the second big factor. If a mini digger can reach the garden through a side passage, excavation is quick. If everything has to be barrowed through the house or over a wall on a terraced street, expect a day or two extra labour and a higher spoil removal cost. Skip permits on Bristol City Council roads add to that if there is no driveway.
Drains and services frequently sit right where the conservatory wants to go. Building over or within 3m of a public sewer needs a build over agreement with Wessex Water, and rerouting a gully or soil pipe can add £400 to £1,500 depending on the run. Manholes inside the footprint need to be raised and fitted with a sealed double lid.
Traditional strip or trench fill concrete foundations remain the most common choice and the most predictable in cost. They suit most gardens where the ground is firm within a metre and there are no mature trees within influencing distance.
Steel base systems, sold as prefabricated frames on adjustable feet, are quicker to install and avoid heavy excavation. They typically cost £2,500 to £5,000 supplied and fitted for a mid sized conservatory, so they are not always cheaper, but they can win where access is poor or the ground is unreliable. They still need proper pads or ground screws beneath them.
Piled or raft foundations are the expensive option, sometimes £8,000 or more for a modest footprint, and only worth it where trial holes show soft or waterlogged ground, or where a large tree is within a few metres. A reputable builder will dig a trial hole before quoting rather than guessing.
Ask for the quote to be itemised: excavation depth, concrete specification, blockwork, insulation, damp proof membrane, floor screed and spoil removal. A single lump sum for the base makes it impossible to compare like with like, and it hides the assumptions about ground conditions.
Check whether Building Regulations apply. Many conservatories under 30 square metres with an exterior quality door separating them from the house are exempt, but if you want to remove that door or open the wall up, the foundations and floor will be inspected. A builder who mentions this before you ask is usually one worth keeping.
Finally, ask what happens if the ground turns out worse than expected. Deeper footings should be priced as a per metre extra agreed in advance, not sprung on you halfway through the dig. Written terms on this point save more arguments than anything else in the job.
The foundations themselves do not need planning permission. Most conservatories fall under permitted development, but conservation areas such as Clifton and Redland have tighter rules, so it is worth a quick check with Bristol City Council before you commit.
Around 600mm to 750mm is typical on firm ground, but on Bristol's clay soils or near trees you may need 1m or more. A trial hole before quoting is the only reliable way to know.
Almost never. Patios and old slabs are rarely deep enough or reinforced, and building on them tends to lead to cracking and leaks at the wall junction within a few years.
Tell us about the job and we will get straight back to you with a free quote. DGL work across Bristol and the surrounding area.
Bristol-based, working across Bristol and the surrounding area.