How deep should foundations be when converting a conservatory to an extension

Question

I am thinking about converting the conservatory into an extension so we can use it all around the year without having dedicated heating in there. Could someone explain how I go about checking the foundations so I can get a closer idea of the costs involved? What depth is required to support an extension instead of a conservatory?

Constantin · Answer

Hi Sharon, 
What Determines the Required Depth?
Building Control will assess:
Soil type
Clay → deeper (shrink/swell risk)
Sand/gravel → usually shallower
Made ground → deeper or piled
Trees & shrubs
Large trees nearby significantly increase depth requirements
Nearby structures
Foundations must not undermine neighbours
Loads
Heavier roof (tiles vs polycarbonate)
Cavity walls instead of frames
Drainage
Existing drains may affect design
What Usually Happens in a Conversion
When the conservatory is upgraded:
Existing floor slab is often removed
New trenches are dug alongside or inside
Proper strip foundations or trench-fill concrete installed
New insulated floor built to regs
Sometimes the existing foundations can be reused—but only if trial holes prove they’re deep and wide enough and Building Control agrees.

Kind regards 
Constantin

Mark · Answer

Hi Sharon, it depends entirely on the ground it's being built on.
An average footing/foundation would be 600mm deep if the ground conditions are optimal.
To find out the depth of your existing conservatory foundation dig an inspection pit up against the brickwork outside. Approx 600mm - 1m square. Dig down till you expose concrete then dig beyond that to find the bottom of the layer of concrete this will give you the depth of your existing foundation.
Normally a standard conservatory wouldn't require such a deep foundation but an extension definitely would.
Hope this helps

Kind regards
Mark

How deep should foundations be when converting a conservatory to an extension

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