Hello. Advice regarding basement build

[Warben]

I’m planning a single-storey rear extension with a basement underneath, and I’d love to hear from anyone with experience of something similar — particularly around costs (£/m²) and who to get involved.

The rear of the property sits on a significant slope, so a retaining wall will be needed regardless. I’m in North Somerset, about 20 minutes from Bristol. The basement doesn’t need to be the same size as the basement. I’ve put a picture of the rear of the property below. 

The main reason for the basement is acoustic separation — I want to use the space as a music room (drums, electric guitar) along with a cinema room or gym. Being below ground should help keep the noise away from the rest of the house naturally.

I’m keeping it single-storey partly because light is already a consideration on this plot, and I don’t want to affect the neighbours’ light either. The plan is to use rooflights to bring as much natural light into the extension as possible.

So — who exactly needs to be involved (specific type of architect, structural engineer, specialist basement contractor?), and what are realistic cost expectations for this kind of build in the South West?

I know basements aren’t everyone’s first suggestion, but I’m genuinely keen on the idea and would love to hear from people who’ve actually done something similar rather than just being talked out of it!

[Nickfromwales]

49 minutes ago, Warben said:

The basement doesn’t need to be the same size as the basement

Ok, lol. Which way around does that need to go and I’ll edit it for you     

 

Go for it, I’m a big fan of basements, and I think this would be a good candidate for an EPS based ICF. Many birds, one stone.

 

Any plans you can share yet, for comment?

[saveasteading]

54 minutes ago, Warben said:

actually done something similar

Need to know a lot more. 

 

Existing house construction.

Which way is the slope

 the gradient.

Are other structures or boundaries nearby.

Ease of access.

What kind of ground.

Size of course.

 

Basements are inherently expensive.

Gut feeling with no info: 3 x normal cost.

 

but perhaps yours is more an extension at lower level. Double?

 

So start with that. 8k/m2 or more.

 

I wouldn't start with any consultant as long as you can sketch to scale.

You sketch your ideas first and put them on here.