Foundations on single extension, are they deep enough to build above?

[ThisMustBeMyDream]

I am considering a house which has a single story extension. I'd like to build above. This is what the vendor has sent from the original plans. I'm awaiting the builder to get back to me but in the meanwhile does anyone know if this is deep enough on the foundations for a 2 storey extension? Thanks 😁.

When we were buying a house with a single storey extension they made a point during a viewing of saying the foundations that were in could take a double storey extension, so there was option to also extend upstairs too.

we had no plans to build up though so not sure if it actually does make a difference.

See if the local council has the Building Regulations plan on file.

It has for many years been common practice for even a single storey extension to require proper foundations adequate for full height

If no proof is available you will have to dig trial holes to expose the founds.

There is a plan on the council website, but it doesn't have the full information. This is what the daughter of the vendor (who is in a care home) has sent today after I enquired with them. They have the full plans.
The extension was built early 2013.

Our double storey extension had to have 2m deep foundations. It would have been 3m if there were trees/hedging close by, as a neighbour discovered.

The depth required for a 2 story extension will probably be impacted by the soil type and what sort of extension you want to add to the existing foundation. Ideally, you would get professional advice from a surveyor who has had a chance to inspect the building and knows what you are considering. Taking short cuts in building works isn't really advisable.

Our foundations on a single story were not.

We had to get a structural engineer to measure the concrete slab by excavating until it was measurable.

This then clearly gives the weight it can take. Maybe regulations have changed over the years. But our extension three years ago was onto existing slab and could only be a single story.

A slab of concrete isn’t foundations at all. Foundations are concrete filled trenches.,The depth is calculated based on ground conditions and size of extension. 3m isn’t necessary and is exceptionally difficult to dig. 2m is often suitable for clay and tree nearby. That’s what we have and DH is a structural engineer. Measure foundations if you cannot get precise details.

Sometimes it is.

www.designingbuildings.co.uk/wiki/Raft_foundation

Yes - raft foundations are perfectly normal!

It sounds as if the OP’s property has strip foundations at least 1m deep, but the advice would be to have a hand dug trial pit to expose the foundations to check.

What passed building regs in 2013 probably won't now. You need to get a structural engineer involved, or look up current regs and see if these comply (which I wouldn't rely on myself - I've always had holes dug to check foundations are as they are purported to be).

Building regs for foundations haven't changed significantly in recent years. But digging a trial put to confirm is useful.

The pressure that they have indicated is decent enough. I would expect that a typical 0.6m wide strip foundation would be fine for a normal two storey extension, but get a structural engineer to confirm.

Building Control are likely to insist on trial holes dug around the extension to check (even if they oversaw the original a few years back).
It's standard practice and can be done by you or by your builder before you begin work.