Foundations for first floor extension

[carbar]

Hi.

I have a single storey extension that was built before I bought my house. It is probably 30 years old and there is no record of it at the council.

I would like to build on top of it and I understand that I might need stronger foundations.

However, before I set out digging inspection holes and so forth, I was wondering if someone could tell me exactly what would be involved in strengthening the foundations? Is underpinning the only option (for example could you build columns outside the existing extension and build the upper storey on those?

Thanks,

Carl

 

[robbie77]

I can't fully answer your question but I can give you an example of what a neighbour of mine did recently.
The foundations he had were only really suitable for single storey but becaue he used a timber frame build for his upper storey he did not have to alter the foundations - maybe to do with the weight saving by not using block and brick.
Might be food for thought.....I'm sure one of the building Guru's can help more

 

[^woody^]

Underpinning is normally the only practical or viable solution

There are other methods but these would be prohibitively expensive and disruptive or may not even be possible

Your idea of columns (on new pad foundations) with associated beams or lintels is an option, but design wise may not be very elegant and not acceptable to planners unless it really is a nice design

Even with timber frame, you would need to expose the foundations for checking, but it is an option - although you may not be able to have a brick external face if the foundations are not very deep

You really need to dig a couple of holes first to check the foundations - one should be where the extension joins the main house to look at the house foundation depth

 

[carbar]

Thanks for the response.

Assuming the foundations do need underpinning, presumably the first task is to dig a trench exposing all of the existing foundations that need underpinning? If so, how wide would this trench have to be?

The reason I am asking is that the access is quite limited, so the trench would have to be dug manually.

Thanks,

Carl

 

[noseall]

Assuming the foundations do need underpinning, presumably the first task is to dig a trench exposing all of the existing foundations that need underpinning? If so, how wide would this trench have to be?

No, don't expose the whole run as this could leave this particular elevation vulnerable somewhat.
Typically for underpinning you would dig 1m and leave 1m, then dig another metre etc. You need to dig a wide trench so that you can excavate sideways under the extension.

 

[^woody^]

The underpinning only needs to be as wide as the existing foundation - unless there is no foundation in which case it will need to be about 600mm wide

You will need at least another 600mm width to work in outside

And you would dig out in bays of 1m, and underpin alternate bays - not dig a trench in one go

 

[noseall]

The underpinning only needs to be as wide as the existing foundation - unless there is no foundation in which case it will need to be about 600mm wide

You will need at least another 600mm width to work in outside

And you would dig out in bays of 1m, and underpin alternate bays - not dig a trench in one go

That's what i just said....

 

[joe-90]

Better off knocking it down and starting again. No-one wants to buy an underpinned house, and you could also build to the new insulation values.

 

[noseall]

Better off knocking it down and starting again. No-one wants to buy an underpinned house, and you could also build to the new insulation values.

Believe me Joey, no one wants to underpin either.

Lousy job.