His an interesting question. Should the foundations on an extension to a house be laid at the same depth as the foundations on the house itself? If not why not?

In sustainability terms, it seems crazy to build hugely deeper foundations which use an awful lot more concrete and produce additional problems and costs of disposing of the spoil when it may well not have been necessary. I think this approach would be better for the environment too. :-)

This has seemed bonkers to me ever since our first project, a lightweight sunroom that had massively deeper foundations than the 150yo stone house it is attached to. Likewise the roof timbers were massively over engineered compared to the original house.


Istm it is because modern occupants and modern lining materials (plasterboard) are less tolerant of flexing and settlement cracks than was historically accepted.


If built to historic standards, our sunroom would have sagged and cracked a little and the window frames would have been a little off square and needed joiner work to make the windows fit and open, and the plaster cracks and paint would need repairs after a few years.

This probably also reflects that concrete costs are cheaper than they were in 1850 and labour costs are greater.