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?

Omid · Answer

Depends on area and council 
  We done 1 meter, in this job I’m in  we done 150cm

Kevin · Answer

Generally 1meter deep is required as a minimum 
If there are drain man holes and  they are deeper within 500 mm some say 800mm you would need to go deeper on foundation depending on depth of drains as could impact pipe work 
Also ground make up 
Building control would be needed to assess when excavation has been completed

James · Answer

It depends on ground but at least a meter down and more remember its all in footing must be solid ground with climate change you must think about long dry spells because if the footing is not deep enough ground dry out and then the dreaded subsidence. and it wont happen straight away can take a few years to be found out go deep get it inspected by building control happy days. Dig by the side of existing footing and see how deep the base is and what depth the concrete footing is should be eight inches plus and remember you want it 600 mm wide

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

John · Answer

Short answer is you don't know until you start digging. Need to hit virgin soil. Building inspector will determine that.   Local drains will also determine depth. Below if there close. 45 degree angle if further out. It depends where you live . Around Birmingham area 800 - 1000mm normally hits clay, shale even sand.

Darren · Answer

Hi Sharon, for an extension the foundations are to be a minimum of 1m but will increase if ground conditions and other factors dictate. The conservatory foundations are typically 500mm  to 600mm deep, so if not to required depth will need breaking out before excavation for the extension foundations. 
D WHITE BUILDERS LTD

Barry · Answer

Normally footings are based on 1mtr from ground level. BUT this is all dependent on ground conditions and subject to building control approval. Things like trees can and do have an effect on design.

Ben · Answer

Depth of dig should be at least 1m deep x 600mm wide
With a minimum thickness (height) of concrete 600mm

Dan · Answer

To convert your conservatory into an all-year-round extension, the first priority is verifying the existing footings. Most conservatories sit on shallow foundations (approx. 600mm), but a permanent extension requires a minimum of 1,000mm to comply with Building Regulations and support the extra weight of masonry and insulation.
 
I would suggest a 'Trial Pit' inspection as a first step. We dig a small hole to measure the depth and width of your current concrete base. This allows us to confirm if we can build on what’s there or if we need to budget for underpinning. Doing this now prevents any 'hidden' costs once the build starts and ensures the project can be fully signed off by Building Control.

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

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