Foundations for Extension

[m13eys]

Hi Guys,

 

Am looking to put a 8 x 4 meter extension on my new build property. I have got the architect drawings done and I’ve gained permitted building rights and planning permission. I have requested building control and currently getting structural calculations done. 

I am looking to take the whole back wall out which is 8.7m with cavity. So I am looking to take out 8.1m of the back cavity wall and put a steel across. I understand this will need to be spliced. Is it possible to use a 300x300 beam on engineered pad stones on the 300 non each side? 

Also it turns out that the house foundations of the house is 2.2-2.7m deep traditional trench. Do I need to go to this level? Also am being respirated to get a survey done by geotechnical to see what’s down at that depth but this is a crazy amount of money. I can assume that the ground is good as my house is already built. Is it a waste of money to get these surveys done? 
 

In a nutshell 2 questions really?

 
1. Can I use a sliced 8.1m 300x300 steel on engineered pad stones supported on 300 nib? 

 

2. Depth of footing for extension, do I need to go to house depth and is a survey required? 
 

Thanks in advance. 

[Conor]

Correct answers:

 

1. That's for your SE to decide.

2. That's for your SE to decide.

 

Guesswork:

1. For that kind of opening, highly unlikely a single steel resting on existing 300mm nib wall will do, I'm almost certain your SE will spec a letterbox (portal) frame.

2. Dig a trial hole at the corner of the house with your SE present, they will then determine. Cost less than £1k. If this is a relatively new build, then I assume the footings were out in at that depth for good reason and BC will likely request you do the same.

[Dave Jones]

Conor is correct regarding both items, in my area building control are only insured for foundations unto 1.5m deep. Deeper than this requires another firms indemnity policy to cover it, e.g. Structural Engineer.

[m13eys]

Okay great thanks for the advice 

 

the structural engineer has came up with a few different options. Looks like I might have to get my learners plates for the digger out!