Creating Basement below new Extension

[ln1234]

I am looking to build a rear extension to my house, and was thinking about the feasibility of creating a basement under the new extension. The new extension's floor would be about 1m off the ground (to be in line with the existing house floor) and i'd probably need about another 2m dug down for the basement.

How much more effort/complexity/cost would be added on to the extension project by incorporating a basement? I've done a search on the forum and the web but couldn't find a similar project being undertaken.

From the picture below you can see the house is built on a slight slope and the rear extension would be about 1m off the ground before the floor starts. Any advice on whether this is possible, and things I should consider?

Cheers

 

[noseall]

It may be possible yes.

But, i would be considering what effect the excavations may have upon the existing building and using the services of a structural engineer to design the restraining walls.

Also consider using a decent company that specialises in subterranean buildings and all the waterproofing that goes with it.

 

[^woody^]

And don't forget that this will need planning permission, before you get to involved in planning anything else

 

[Shytalkz]

And ,to save BAS saying it, once you have planning:

You must of course, comply with the requirements of the Party Wall etc Act 1996, if applicable.

 

[ln1234]

Thanks for the replies. I will definately be using a structural engineer to design/calculate the structure, and will be getting planning permission and complying with building regs etc. Party wall act is something I need to look into as well.

What I was really hoping for are some opinions on whether this would be a viable project, and how much extra effort/cost would be required to create the basement. The ground floor of the existing house is quite high up (steps are needed to get into the garden).

Cheers!

 

[Shytalkz]

Structural things to think about:

• the need to uderpin the existing foundations to allow for the increased depth;
  the type of soil that you are on which will determine foundation type;
  is there a perched water table;
  retaining walls to the sides/rear;
  are there adjacent buildings that will require stabilising.

It will obviously cost you some more than just keeping to existing floor levels and had an unuseable void underneath. How much more? Impossible to say without knowing the exact circumstances, but, assuming no ground difficulties or adjacent owners to foul things up, an extra £25-30k should more than cover things.