Depth of foundations required on existing footprint of conservatory

Catbells

Hi I'm having a conservatory and kitchen combined into one room.   Currently the conservatory is constructed primarily of glass with a half-height brick wall.  The new building will be on the same footprint as exists currently but will be full height brick wall and bifolding doors at the front.  
 The total depth of existing foundation and brickwork is 70cm.  The  brickwork is 14cm  and the remaining foundation is 56cm.   I need to find out whether the depth of the existing foundations will be deep enough for a more solid structure.  Any help as to how to find out would be very helpful.  Thanks.

FreeBear

Catbells said: I need to find out whether the depth of the existing foundations will be deep enough for a more solid structure.  Any help as to how to find out would be very helpful.

How deep the foundations are is easy to check. Just push a steel pin into the ground next to the brickwork and see how far down it goes. But for a proper extension, you also need to know how wide the foundation is and also how thick the concrete is. The only way to discover this, is to dig a small pit and expose the concrete at the bottom.

stuart45

Is the 700mm the measurement from the ground level to the bottom of the concrete foundation, 560mm the depth of the concrete, and then 2 courses of brickwork from the concrete to ground level?

DRP

We are doing exactly the same right now (builders working outside my window)

Building control asked us to dig a trial hole on both elevations (conservatory was an infill with 2 existing walls and 2 new).

 The concrete was 750mm deep in the ground (about 500 concrete in height) and 200 to 350mm from the face of the wall. BC were happy to accept this as sufficient for our single storey extension.

Catbells

stuart45 said:

Is the 700mm the measurement from the ground level to the bottom of the concrete foundation, 560mm the depth of the concrete, and then 2 courses of brickwork from the concrete to ground level?

Yes that's exactly it.

Catbells

DRP. I note you spoke to building control at your local council.  I wasn't sure I could do this and instead thought I had to employ someone to come round to look at it and calculate.  

Freebear - I have had a hole dug by the side of the existing wall and measured the depth of the concrete and brickwork. Haven't yet had the thickness measured so thanks for that information.

stuart45

The decision rests with the Building Inspector, so although people can give advice, it's the BCO who really decides. It depends on the type of subsoil you have, amongst other things. 
It also depends on how the footings were constructed. For conservatories the people doing the base would often build only the outer skin on the foundation, go up to DPC, lay the slab, and then build the inner skin off the slab.For an extension both the inner and outer skin should sit on the foundation.
Are you hoping to keep the floor slab as well?

Catbells

Just to clarify its not an extension - the build will follow the old footprint.    I don't know what the floor slab is.  But I think it might be the existing floor?   If so  No - we are levelling the floor as currently the kitchen floor is slightly lower than the conservatory floor by about 3 inches.  Which makes me think now that the few inches we are lowering the conservatory floor to match the kitchen might make all the difference.  Lowering the cons floor may  make the existing foundations too shallow.

Section62

Catbells said:

Just to clarify its not an extension - the build will follow the old footprint. 

Although you aren't making the building larger, what you propose is an 'extension'.

The rules on conservatories are quite relaxed compared to dwellings, in part because conservatories are thought of as having a limited life span.

Getting rid of the conservatory and replacing it with a structure with 'proper' walls and a roof means you are building an extension - and consequently you'll need to follow the rules that apply to extensions.

E.g. you may need to apply for planning consent, even though the extension is replacing and existing structure.

DRP

Catbells said:

DRP. I note you spoke to building control at your local council.  I wasn't sure I could do this and instead thought I had to employ someone to come round to look at it and calculate.  

Freebear - I have had a hole dug by the side of the existing wall and measured the depth of the concrete and brickwork. Haven't yet had the thickness measured so thanks for that information.

Council BC made the decision after coming to look at the trial pits. They told me if it was from scratch they would normally ask for 50mm more depth, but the conserv had been there 20 years without movement so was unlikely to struggle with the small amount of additional weight of a single storey extension.

Prior to that, I spoke to (but didn’t engage), a structural engineer. I don’t think they could really offer liability or a huge amount of reassurance via calculations,as there would always be a certain amount of guesswork over how the foundations were put together. 

Catbells

Section 62 thanks.  Thats so interesting to find out.