We have had several quotes for a 4.8 x 4.0m conservatory – all specify 1m foundations. (Permitted development outside pp and building regs.)
We are on clay soil and there’s a nearby mature oak tree with TPO. Fortunately the rear wall of the conservatory is just outside the ‘root protection area’. Nevertheless the tree coupled with the high volume change potential for the clay soil would make 1m foundations wholly inadequate which surprises me given that all the conservatory companies are large, established specialist businesses offering a 10 year guarantee.
Using this:
I’ve imputed the data and the answer comes up as “Foundation depth in excess of 2.5m requiring structural foundation design.” Further I’ve spoken to a structural engineer who has recommended 2.7m trench filled foundations !
This depth seems excessive and potentially prohibitively expensive especially given the comments here under ‘foundations explained’ -“However, there comes a depth (around 2.5m deep) beyond which it becomes impractical and dangerous to work.”
Moreover can new foundations be excavated possibly below the existing house foundations albeit at right angles to, not adjacent to ?
Any help would be much appreciated.
(PS Have contacted my local building control who haven’t responded)

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