Underpinning

[scotal]

One for the Ph builder types, we are currently looking at a bungalow that has permission to change the loft space to 4 bed/2bath living space. However we've been told that the whole property will need underpinning to take the extra strain.

Couple of questions;
1. Is this normal?
2. If so, how much does underpinning cost. we've been told to allow for 14mx16m underpinning.

Thanks.

[jamesuk28]

Under pinning is normally required if the current building's foundations are not deep enough to take an extension. Or the building is subsiding.

In answer to you second question it is bloody expensive. The founds will have to be dug out, rams inserted and then backfilled. Last quote I got for approx 3m x3m was knocking £14,000 However it depends on what your founds are sitting on (this was clay) and the quote was by a builder renound for being very very expensive.


Get a builder round for a quote.

[cj.ok]

scotal said:

One for the Ph builder types, we are currently looking at a bungalow that has permission to change the loft space to 4 bed/2bath living space. However we've been told that the whole property will need underpinning to take the extra strain.

Couple of questions;
1. Is this normal?
2. If so, how much does underpinning cost. we've been told to allow for 14mx16m underpinning.

Thanks.

A trial hole will probably need to be dug to see what foundations are currently in place... if Building Control are notified of your works they may insist on this (it is pretty cheap though). If you were building another storey on top of say a flat roofed garage then YES i'd say you'd need underpinning, however you're not adding that much extra load so you may get away with what you have already.

[billsnemesis]

jamesuk28 said:

Under pinning is normally required if the current building's foundations are not deep enough to take an extension. Or the building is subsiding.

In answer to you second question it is bloody expensive. The founds will have to be dug out, rams inserted and then backfilled. Last quote I got for approx 3m x3m was knocking £14,000 However it depends on what your founds are sitting on (this was clay) and the quote was by a builder renound for being very very expensive.


Get a builder round for a quote.

Just out of curiosity J where are you based?

I am getting plans drawn up to dig out the cellar beneath my house (as well as a lot of other work) and am looking at figures of around £1k per cubic metre. We believe most of the foundations go down about 2 metres anyway so we need another half metre to give us a decent ceiling height - maybe call it a full metre to allow for new footings. We will have about 40m of wall to be underpinned.

If we can get the cost below £1k per linear metre for underpinning it should be financially viable - otherwise its borderline and will eat into the budget for the remainder of the work.

Your figures are interesting but we are in SE London so I would like to see whether there is a regional variation or whether you are getting much better prices than we have found so far. We are on stuff that I would describe as sand, not sure what that does to the cost but the houses have been there for over a century and don't seem to have moved much so it seems to be reasonably stable

[jamesuk28]

billsnemesis said:

jamesuk28 said:

Under pinning is normally required if the current building's foundations are not deep enough to take an extension. Or the building is subsiding.

In answer to you second question it is bloody expensive. The founds will have to be dug out, rams inserted and then backfilled. Last quote I got for approx 3m x3m was knocking £14,000 However it depends on what your founds are sitting on (this was clay) and the quote was by a builder renound for being very very expensive.


Get a builder round for a quote.

Just out of curiosity J where are you based?

I am getting plans drawn up to dig out the cellar beneath my house (as well as a lot of other work) and am looking at figures of around £1k per cubic metre. We believe most of the foundations go down about 2 metres anyway so we need another half metre to give us a decent ceiling height - maybe call it a full metre to allow for new footings. We will have about 40m of wall to be underpinned.

If we can get the cost below £1k per linear metre for underpinning it should be financially viable - otherwise its borderline and will eat into the budget for the remainder of the work.

Your figures are interesting but we are in SE London so I would like to see whether there is a regional variation or whether you are getting much better prices than we have found so far. We are on stuff that I would describe as sand, not sure what that does to the cost but the houses have been there for over a century and don't seem to have moved much so it seems to be reasonably stable

Hi we are based in Northants. <£1k / m might be possible given you have 40m to do. Like I said it will vary from builder to builder. Get your local building control / planning bod down from the council. Our founds went down 1.7M the council required a minimum depth of 2m. However the chap that came out to see us (from the council) was old school. He had a quick look down the test hole, said "well it should be a bit deeper but seen as the building has not moved in 30 odd years, you'll be Ok, fill it in and get cracking"

If it had been a young chap come out I can guarantee he would have made us underpin costing me an extra £14k. So cross your fingers you get somebody from the council who uses their common sense.

Edited by jamesuk28 on Monday 19th February 15:03

[billsnemesis]

I think with planning officers you have to cross your fingers anyway. Some friends of my parents built a single storey extension recently and were told they need concrete footings six feet deep

You could have built St Pauls on that!

I suspect that in London the specialists in cellar construction look at the amount of value it adds to the house and take a proportion of that rather than the actual cost in terms of labour, time, disposal of waste and cost of materials

You position is useful as a benchmark though.

[Sam_68]

scotal said:

Couple of questions;
1. Is this normal?
2. If so, how much does underpinning cost. we've been told to allow for 14mx16m underpinning.

1) No. It indicates that you have very marginal foundations, if they aren’t able to take the additional (minimal) loads of a loft conversion.
2) As others have said ‘it depends’. Could be anything from underpinning with conventional strip footings (expensive), via a underpinning with a reinforced ring beam (very, very expensive) up to fancy composite designs with ring beams/piles etc, to deal with very suspect ground conditions (cheaper to demolish and rebuild!)

You’ve been told by who, exactly, that you need underpinning, and on what grounds (pardon the pun)?

Jamesuk28 said:

Under pinning is normally required if the current building's foundations are not deep enough to take an extension.

Foundation depth isn’t the usual problem (unless the original foundations haven’t been taken down to suitable load-bearing strata) – normally it’s width. The job of the foundations is to spread the load onto enough area of subsoil to support a given weight…if the subsoil is weaker, you increase the width of the footings, up to a certain point. If the subsoil is really weak, you have to look at things like raft or piled foundations.

Foundation depth is usually either to get down onto stronger underlying subsoil (ie. through the easily compressible vegetable topsoil) or to be deep enough to protect the concrete of the foundations from heave or frost damage.

Modern strip footings are usually at least a couple of feet wide. Some older houses don’t have strip footings at all… they may simply have the wall built off subsoil, with no real foundations to speak of.

billsnemesis said:

I think with planning officers you have to cross your fingers anyway. Some friends of my parents built a single storey extension recently and were told they need concrete footings six feet deep

Planning Officers have nothing to do with foundations. Planning Officers deal with Planning… which is about aesthetics, amenity and land use.
Allegedly.

The Building Regulations (which is the legislation that dictates structural design, including foundations) is administered by Building Control Officers, who are a different breed altogether.

Some of the younger BCO’s can be a bit cautious and play it by the book, but many of the older ones are extremely sensible and pragmatic and know their area intimately.

[Tony427]

Rather than underpinning the walls have you thought about putting in a steel framework built inside the existing house, to take the loads of the new floors above.

A mate had exactly the same problem as you ( wanting to extend up into a bungalow roofspace but with inadequate foundations for the extra weight) and successfully got round the problem by putting in a supporting steel frame built from just a few concrete pads built through the existing floor into decent subsoil below. Once in and with the steel supports boxed in you wouldn't know the build is unusual at all.

This building system also enabled him to have larger rooms than normal so under a couple of bedrooms he's treated himself to a snooker room.

Just a thought.

Cheers,

Tony