Do extension foundations need to go to house foundation?

[m13eys]

Hi 

 

The UK standard for trench foundations is 1000mm deep x 600mm wide. Is this the case for all extensions or do you need to meet current house foundations? 
 

cheers 

[Conor]

Depends on your ground conditions, how your extension will or won't tie in to the existing, and the whims of your building control officer. If you have an SE involved in your project, then they can advise. And it will be difficult for the BCO to argue against a spec that had been designed and signed off by an SE. A trial pit is really useful.

Edited March 18, 2023 by Conor

[nod]

As a rule BC will normally want the foundations at least the depth of the house 

 

I currently building an extension Ground conditions are fine 

But the house was built four years ago using piles 

Hes insisting on piles for the extension Without understanding why the house was piled 

[joth]

2 hours ago, Conor said:

and the whims of your building control officer.

This.

 

Ours signed off a raft foundation for the extension, and only after it was dug changed his mind and wanted strip foundations deeper than original and tied into the old dug out. The excuse was "because of soil conditions" but when challenged he agreed they were basically as good conditions as could be hoped for and there wasn't any other ground that could have changed his mind.

I.e. he never properly looked at the submitted plans thinking he'll just decide after they're dug.

Cost of£5000+VAT in unnecessary muck away. But I did gain +100mm of extra floor insulation for it.

[ETC]

Strip foundations for an extension should - in most if not all case - go to the bottom of the foundations of the dwelling.

 

If you think about it logically - if the foundations of a house are taken to a certain depth to get good ground why wouldn’t the extension foundations go to the same depth?
 

I’m sure a structural engineer would be able to justify an alternative option.