Extensions Question

How can we build over an existing ground floor extension which has inadequate foundations?

We want to buy a house with a ground floor extension of approx 4.7 x 10m which was built in the late 1970s. Our aim is to add more bedrooms. We suspect the foundations aren't strong enough & the vendors won't agree to digging inspection pits. What are our options to extend upwards if the foundations aren't good enough please? And what are the likely costs in a worst case scenario situation?

6 Answers

Best Answer

worst case scenario demolish and rebuild ,might be able to underpin existing,underpinning allow 8-10k rebuild allow 70k plus depending on spec/scope of works roof structure ect

Answered 18th Jun 2011


Member since 7 Jun 2008

Hi your cheapest & quickest option is to knock it down & start from scratch, underpinning is time consuming & expensive. You might be lucky when you can dig a test pit. Costing will depend on many variables including your type of finish i.e kitchen, bathroom etc.

Answered 18th Jun 2011


Member since 15 Jun 2009

hi lee complete building there a few ways to do this steel work up and over with concrete pads to the right depth or pillars on the inside of the old extension to pick up steelwork.

both ways are not cheap

hope this helps

Answered 18th Jun 2011


Member since 17 Nov 2008

I can understand the vendors not letting you dig trial pits, as you havent yet committed to buying the house.
Only option is if you do buy the property, is first dig some trial holes to accertain the depths/thickness of existing foundations, engage an architect and structural engineer, they will then advise if foundations need underpinning.
There are now more modern methods for underpinning foundations.
Your architect and s/e will advise and all needs to go through planning and building regs.
Cant advise on the cost as the scale of works is unknown at present, the foundations might even be ok.

Answered 18th Jun 2011


Member since 29 Oct 2008

You either have to underpin the existing, or pull it down and start again. Underpinning can cost you an arm and a leg.

Answered 18th Jun 2011

Rupert Burrows Design Contracts

Member since 16 Nov 2008

Ask a structural engineer.
It may be you have either to underpin the existing foundations or another way could be using Universal beams and Stanchion to make a steel frame to support the extension above A structural engineer should be able to help with this and also supply plans and calculations for the building control officer (BCO)
Cost is expensive depending on what would be specified


Answered 18th Jun 2011


Member since 13 Jan 2010

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