width of footings - extension

theGrinch

I am building off a party wall (neighbour extended in 2015). Therefore I need to allow for block and insulation on my side.

The drawings have the footings width on this side at 500mm. The builder has said he thinks 300mm is fine.

Typical, I cant get got of the architect/structural engineer this morning.

Any ideas?

Doozergirl

Well, you'd normally build a 300mm wall with 600mm footings, giving 150mm either side. I'd be surprised if you can get to a metre depth easily with only 300mm width. A small bucket on a digger is 450mm, so I'd be inclined to go with that. 500mm is awkward unless you're hand digging.

The person with the final say is the building inspector.

Has your architect provided plans for full approval to building control or is the builder doing it under building notice?

If it's under plans, then he'll need to build 500mm or it will be a world of admin work to change them.

If it's under building notice, then you can talk to the building control officer and ask what they are happy with.

System

sounds like you have already involved a structural engineer, presumably they have designed the foundation?
450 is generally as small as a foundation would go for a single leaf wall (as you generally have 150 either side of the subfloor walls)

theGrinch

hand dig.

building notice

Doozergirl

Then call the building inspector. He will dictate.

teneighty

How deep is this foundation?

Hand digging a trench 300mm wide any deeper than 450mm will be rather challenging as you've got no room to stand in the trench.

I would say 450mm wide is the minimum just based on practicalities of digging it by hand or machine.