Building on old extension - Rebuild or Underpin?

smg0102

Hi all, 

We have had planning approved for a double storey side extension, within part of it has us building on top of our existing single storey exension to put a bedroom on top. The extension was build before we bought the property, 30 years odd ago, so we've done a pilot hole and the foundations aren't sufficient, which isn't a surprise. 

We have got builders quotes and are down to two; the cheaper is proposing underpinning the original extension, whereas the more expensive is proposing knocking down and rebuilding. 

Would really welcome thoughts or experiences on either of these approaches if anyone has them!?
Thanks in advance
Steve

Section62

smg0102 said:

We have got builders quotes and are down to two; the cheaper is proposing underpinning the original extension, whereas the more expensive is proposing knocking down and rebuilding. 

Would really welcome thoughts or experiences on either of these approaches if anyone has them!?

Have you had any guidance from building control on what they would find acceptable?

As well as structural strength, older walls may not have the same levels of thermal resistance/insulation as modern construction.  If it were me I would take into account the possibility that a slightly higher capital cost of rebuilding the wall may return savings on future energy consumption.

TBH I'm slightly surprised the underpinning quote is coming back cheaper. Although you 'save' the wall, the faffing around doing the job properly can easily cost as much as the extra materials and labour needed for a full rebuild.  There is also 'risk' involved with underpinning if you find the situation is worse than expected.

Does the existing extension have a flat roof, or do you have a gable wall to deal with as well?  Will the new external walls be finished brick, or will they be rendered?

Slinky

I'd also consider the U word when it comes to future insurance of your property. Our current house had a side extension many years ago which had subsidence and required underpinning. This was sorted in 1990. We have limited options when it comes to getting buildings insurance as most regular insurance companies won't touch us with a bargepole, despite the non-recurrance and time that has passed.

Obviously you don't have the subsidence issue to consider, but you will be asked about underpinning and may get tarred with the same or similar brush to a property that has had subsidence.

I'd knock it down and start again, which is what we did with our previous home when we had the same footings issue with our garage.

csal

Hi Steve,
I am in a similar position to you where I have planning permission to build above an existing extension with insufficient foundations and am debating the underpin vs. knock down and rebuild route. Do you know if you need to get any additional permission from the planning department to knock down and re-build (vs. underpin and build above)?
Many thanks,
C

Section62

csal said:

I am in a similar position to you where I have planning permission to build above an existing extension with insufficient foundations and am debating the underpin vs. knock down and rebuild route. Do you know if you need to get any additional permission from the planning department to knock down and re-build (vs. underpin and build above)?

If the planning application specified anywhere that the new work would be built on top of the existing structure then it would be wise to contact the council and tell them that due to the condition of the existing you now intend to demolish and rebuild, as technically what you'll be doing is different from the approved plans.

As long as you are rebuilding to the same footprint and the replacement building will look the same, you shouldn't have too much difficulty in getting agreement from the council, the probable worst case is they may ask you to make a formal application to vary the plans.

But if you do need to get formal approval for the variation it may be worth considering whether you should also get permission to change the footprint at the same time - if you are now building from the ground up then making the extension slightly larger won't add much to the costs.  In fact there is the potential for savings if the new foundations can be dug in virgin ground rather than having to get the existing concrete footings broken out and disposed of (which would be necessary to allow the walls to go back where they currently are).

ThisIsWeird

smg0102 said:

Hi all,
We have had planning approved for a double storey side extension, within part of it has us building on top of our existing single storey exension to put a bedroom on top. The extension was build before we bought the property, 30 years odd ago, so we've done a pilot hole and the foundations aren't sufficient, which isn't a surprise.
We have got builders quotes and are down to two; the cheaper is proposing underpinning the original extension, whereas the more expensive is proposing knocking down and rebuilding.
Would really welcome thoughts or experiences on either of these approaches if anyone has them!?
Thanks in advance
Steve

Hi Smg.

Interesting points raised above, and it would almost certainly help your decision - and folk on here to advise - if you consider them if you haven't already done so;

1) How much wall from the existing would be retained, and would require this underpinning? One side stretch? Two?

2) If you were to go the knock&rebuild route, could/would you take advantage and even slightly increase the overall footprint at the same time? How appealing would this be to you?

3) Are the two options from two different builders? No single builder is offering both options?

4) What are the actual differences in quote?

On other issues brought up above, I don't think that future insurance will be an issue at all as you aren't suffering from subsidence, so once this is built and signed off, it'll be a bona-fide build for regular insurance cover. And regarding insulation values, yes the existing walls will undoubtedly be at a significantly lower level, but to bring them up to current levels - as you should as part of this - will be dead easy and not expensive, assuming that the ground floor area will be gutted as part of this work in any case. 3-4" of PIR fitted internally should easily do this, so the only real consequence will be that loss of internal space.

FreeBear

ThisIsWeird said: 3) Are the two options from two different builders? No single builder is offering both options?

If the differing opinions are coming from builders, you need to get qualified (and paid for) advice from a structural engineer. Good foundations are critical for any structure, so you need to get it right first time. Whilst a builder may be able to comment on suitability from prior experience, he/she will have neither the training or qualifications to give a definitive opinion.

Section62

ThisIsWeird said:

...On other issues brought up above, I don't think that future insurance will be an issue at all as you aren't suffering from subsidence, so once this is built and signed off, it'll be a bona-fide build for regular insurance cover.

So what do you do when getting an online quote for insurance and the question says "Has any part of the building(s) been underpinned?" with only a Yes/No option?

As Slinky said last year, it isn't necessarily about being unable to get insurance full stop, rather the potential limitation on companies willing to take on the risk.  Those at the more affordable end aren't interested in taking on more complicated cases and resolving the nuances between underpinning for subsidence and underpinning to extend.  If you can't answer the "Underpinning?" question with a straight 'no' then the algorithm may be set up to say "Sorry, we can't offer you insurance" regardless of all the other circumstances.

On a single storey structure the cost difference between underpinning and demolish/rebuild is rarely likely to depart far from 'marginal'.  Underpinning is labour intensive, and carries a greater risk of finding the situation is worse than expected.

ThisIsWeird

Section62 said:

So what do you do when getting an online quote for insurance and the question says "Has any part of the building(s) been underpinned?" with only a Yes/No option?

How many online insurance sites ask for this, and will allow only a yes/no option? None that I recall. I've seen references to 'subsidence', yes, but never - afaIcr - to 'underpinning'. Realistically, how significant will this 'potential limitation' be? Surely this is a very minor consideration in the scheme of things?

But, yes, I would certainly choose rebuild over underpinning without question, and for all sorts of reasons.

In fact, only a significant cost premium for the rebuild would persuade me to go for the 'pin option - money will talk at some point.

The purpose of my post, as I hope is clear, is to give Smg more of the important considerations when making their decision. I cannot see insurance as being one of them.

gzoom

@smg0102 Our original plans were to under pin existing foundations of our bungalow for a new dormer bedroom. We had pilot holes dug etc etc, but as others have said 



But pretty much as soon as proper works started it quickly became clear demolition and rebuild was the way fowards. Luckily our builder absorbed most of the cost associated with change in plan. Demolition and rebuilt is likely the most staight fowards option for everyone.