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Can someone please advise me how much a Raft Foundation will cost?

Our builders have had to stop work on our extension due to the council not approving 1.2m depth of
excavations due to a nearby tree.

The builder tells me that a Raft Foundation will be a solution - but he also tells me that this will cost an additional £6,000 plus VAT on top of the original quoted cost. This equates to exactly one quarter of the cost of the entire project!

The size of the single storey extension is 6m x 3.5m.

Can someone please advise as to whether these extra costs are reasonable?

Many thanks.

CJW1968,
Suggest you talk to the BR officer and see if he will accept deeper foundations in the area of the tree. Also check he is OK with raft construction in case you have to do this.
end

A raft foundation is not usually suitable where the existing house has a traditional foundation because the raft will move causing a crack at the joint.

The building regs bible for this scenario is the NHBC Tables, from NHBC Guidance note Chapter 4.2, "Building Near Trees"

If you look at the tables you can determine the depth required, but the BCO should have told your builder this already. Identify the tree, and how far away it is from the foundation. Then you need to determine the type of clay, is is low, med or high shrinkability. Unless you have a BCO with local knowledge or have done a soil analysis you will have to assume high shrinkability.

Then using the tables you can determine the depth.

Also if the foundation is over 1.5m in depth, then a void fromer will be required to the inner face of the excavation and a suspended floor will be required.

The high water demand and hence worst trees are things like Poplars, Oak, Eucalyptus, Willows and Lelandii conifer.

I think you really need to have a chat with the BCO and then maybe your architect / engineer.

Good luck.