Under pinning due to extension and not subsidence

[Naz1977]

So we are looking to make an extension on our bungalow. Initially we were going for a full single story extension, however due to foundation and age of property, I decided to go for a dorma conversion instead.

However the structural engineer has now come back and said the existing raft foundations will not pass the building regs for any extra load on them and he has said they need to be under pinned.

Now I know insurance companies don't like to insure if a house has been under pinned, or cost more to insure it, but as this is under pin to extend and not as a result of subsidence, do you think :

1) I may struggle to insure it
2) I may struggle to sell in future if I under pin it?
3) Struggle to remortgage?

On another note, If i am having to pay £15-20k to under pin, then I might as well spend a bit more now and go ahead with the full single story extension!


Thanks for the help in advance

 

[freddiemercurystwin]

No to all three as you're underpinning for an extension not as a result of subsidence.

 

[^woody^]

Underpinning a raft eh? How does that work

 

[Notch7]

Would a single storey extension increase the loading of the house foundation? I wouldve thought the new foundation would be self supporting, unless you are having a big knock through.

 

[CharlesBR]

I am a complete novice but surely any works that need to be done for your works are merely proactive steps to ensure what you want to and not reacting to something that hasn't happened? As long as you follow advice with whatever the structural engineer has recommended and done what the building inspector has said you must do without cutting corners you won't have a problem.

 

[Naz1977]

Underpinning a raft eh? How does that work

The overhang on the inside wall is obviously enough as it is a raft. It is the outer overhang which is only 100mm and structural engineer said it needs to be 200mm. So will need to dig down to 1m below surface and then go under the raft and fill in with concrete.

But I am not a builder nor an structural engineer. So will have to follow them as don't want the house to tumble down on us!

 

[Naz1977]

Would a single storey extension increase the loading of the house foundation? I wouldve thought the new foundation would be self supporting, unless you are having a big knock through.

As said, I am not sure and don't know the ins and outs.
He said the overhang is not much and it could fail.

As part of the building, the inspector would want a report from them to confirm the existing foundation would take the extra load and he said putting all the calculations in, it is not passing the criteria and the system keeps failing it!

 

[Notch7]

The overhang on the inside wall is obviously enough as it is a raft. It is the outer overhang which is only 100mm and structural engineer said it needs to be 200mm. So will need to dig down to 1m below surface and then go under the raft and fill in with concrete.

But I am not a builder nor an structural engineer. So will have to follow them as don't want the house to tumble down on us!

I had a job where the SE specified underpinning, it was a Victorian house with a spread footing on just a bit of clinker.

The engineer wanted the underpinning to be 1 metre x 1 metre. The way it was done was to dig out 1 metre, leave 1 metre, dig out a metre. Then fill with concrete and when set, dry pack the top. When the first batch was dry, the alternate areas were done. I cant remember whether we had to connect the day joints with rebar - I think so. The cost wasnt horrendous, a few £k I think.

 

[Naz1977]

yes, this is exactly what we have to do. They dig alternate 1m long x 1m deep holes and then fill it up with concrete. Once set then goes to the other alternate holes.

It is not a complicated job, but a very labour intensive work I guess.

I am dreading the cost. Somewhere I read you have to allow £1000k per m. and it is about 20m!

 

[^woody^]

The overhang on the inside wall is obviously enough as it is a raft. It is the outer overhang which is only 100mm and structural engineer said it needs to be 200mm. So will need to dig down to 1m below surface and then go under the raft and fill in with concrete.

But I am not a builder nor an structural engineer. So will have to follow them as don't want the house to tumble down on us!

Hmmm. No, the point is that a raft works by floating - spreading the load over a much wider area of ground and moving up and down as one with that ground.

When you underpin and specifically part underpin then that defeats the purpose of the raft and could do more harm than good if anything at all.

 

[Naz1977]

Not sure mate. This is what I have been told. Are you saying I need to get another opinion?

 

[Doggit]

Definitely. What's the ground type there, and are there any trees nearby. Clay swells and shrinks with the weather changes, and trees will suck more water out of the ground in the summer, so a raft will move as one complete unit as the ground conditions change - although it may not just be clay that dictates a raft. The internal walls are also supported on the raft, so how are they going to be dealt with.