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Underpinning for renovation (not subsidence!) - yes or no?

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We are planning a major remodel of our end terrace Victorian house, which would essentially transform the ground floor into one open plan space. This would involve knocking through what used to be the rear wall of the original ground floor, and installing an RSJ and two vertical posts to support the floor above. 

We just received the RSJ calculations from a structural engineer, with the following comment: 

"We have designed the frame with stiff moment connections at the top to avoid the need for large underpins.  However if your builder finds the existing foundations are very poor and shallow, underpins may be required anyway (Builder to check that existing foundations are at least 500mm wide and 600mm deep. If not, allow for provisional 900x900x300thk GEN3 underpins, under both posts)"

This potential underpinning concerned me, as far as both resale and insurance is concerned. However, having spoken to my home insurance provider, I was reassured that it wouldn't affect my premiums, as long as underpinning is done for remodelling, rather than subsidence reasons. They said they deal with these situations a lot, as old houses were built to different standards.

But what about trying to sell the house in the future? Will we be shooting ourselves in the foot, and turning off the majority of prospective buyers, who will hear "underpinning" and run the other way? Or is there enough precedent for this kind of work for buyers to differentiate between remodelling- and subsidence-related underpinning?

I've also come across some posts on this forum warning against the dangers of "partial underpinning" (i.e. only one side of the building, which is what we'd be doing if it turns out our existing foundations are not sufficient), which has apparently been known to put stress on the building and lead to future problems. Again, I'm not sure if this is only an issue with underlying subsidence, or if it's a problem in and of itself. 

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8 minutes ago, dougless said:

I would be more worried by a major remodeling of an old house if foundation strength had not been factored into the calculations.

Hi dougless, and thanks for your comment! I didn't mean to imply that we will proceed with the remodel anyway, and just not underpin (if it turns out the existing foundations are not deep enough). The pertinent question is, do we continue with this project at all if the underpinning is likely to decrease the the market value of our home? Obviously we want to add value, and hopefully recoup some of the remodelling costs when it comes to selling the house.

Our builder and architect say it's very common to underpin old properties during remodels, but I've been struggling to find any info on how it would affect future resale. People are frequently advised not to buy underpinned property, but most discussions just focus on subsidence risk and don't mention remodelling. 

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The joke is that a modern underpin will make it far stronger than it was originally. 

How's a buyer going to know ? 

You get planning permission and building completion certificate for the extension. That's it.

If buyer asks about underpinning you say " rely on your own survey". 

If building regs is happy, that's all that's required. 

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2 hours ago, 24gray24 said:

How's a buyer going to know ? 

You get planning permission and building completion certificate for the extension. That's it.

If buyer asks about underpinning you say " rely on your own survey". 

If building regs is happy, that's all that's required. 

24gray24, thanks for your input! I agree that the stigma around underpinning is ridiculous. But, my understanding was that in the UK the vendor was *legally obliged* to declare if the property had been underpinned, so we won't be able to sneak this under the radar by not disclosing. Unless you know of any exceptions to this?

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1 hour ago, House Cat said:

24gray24, thanks for your input! I agree that the stigma around underpinning is ridiculous. But, my understanding was that in the UK the vendor was *legally obliged* to declare if the property had been underpinned, so we won't be able to sneak this under the radar by not disclosing. Unless you know of any exceptions to this?

How will you know it needs underpinning ? .

you put the job for tender and someone quoted, you accepted you don`t know any different they just did the work. 

 

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Don't underpin the existing foundation. Locally rebuild the affected area on new foundations if you are concerned. For me this is simply part of a structural alteration rather than underpinning in the traditional sense of the word. 

The engineer's proposal makes assumptions with respect to the grounds bearing capacity. If it turns out to be lower than expected a more substantial foundation may be required. If you are concentrating load by making structural alterations you should routinely expect to have to spread the load over a wider area. 

From an insurance company perspective this would not be a question that is typically asked. Insurers generally ask whether the property has ever suffered from subsidence. 

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I'd be going back to your engineer and asking them to design a foundation pad for under each upright. He'll squirm as they will want to know ground conditions below, but will just then recomend a CBR that needs to be acheived. 

Then your builder will be able to quote accurately as he will know the full scope of the job, and not have a possible addition of under pinning. Plus it removes your reliance on 120+ year old concrete and ground conditions. 

And it would remove any doubt about the works come selling the house or for insurance

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21 hours ago, House Cat said:

24gray24, thanks for your input! I agree that the stigma around underpinning is ridiculous. But, my understanding was that in the UK the vendor was *legally obliged* to declare if the property had been underpinned, so we won't be able to sneak this under the radar by not disclosing. Unless you know of any exceptions to this?

I don't think that's true, legally. The owners often have no idea in practice.  The buyers are not allowed to ask questions about things a surveyor should pick up. After all, are you qualified to answer questions that would take an engineer making calculations to answer? 

All I would do is keep all the drawings and design stuff and send it all over to the buyer when you sell. 

Then they can say " that's nice. Where's the building completion certificate? " and then throw it in bin. 

That's how it actually works. Everyone relies entirely on 1. Building control and 2 surveyor. For construction issues. 

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Local underpinning would concern me on a Victorian house. They were built with lime mortar which allows some movement or time without cracking if ground conditions change. Modern buildings are built very stiff. Having localised stiffness where the rest of the house foundation may move slightly could cause problems.

At the back of the house have you got a suspended timber floor with a crawling space? or are they solid floors? or even cellars? most tend to have a solid floor at the back of the house.

My parents Victorian semi has a brickwork foundation that gets wider as it goes down. It might be worth doing a small localised dig down to see the depth of the foundations (and if they step out wider than the wall thickness) before you go any further.

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Won't the buildings inspector make the final decision on what spec the foundation will be?

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