Garden basement "roof"

[jayc89]

Our extension plans include a rear extension to the house and a new, detached, double garage some 16m away from the rear of the house. Between the two will be a "Kitchen Garden", raised planters for veg/herbs etc. 

 

It's probably pie in the sky stuff, I have no idea how much it would cost, even with me doing most of the work, so I'm musing right now, but could a basement run from the new extension (remaining 45 degrees away from the existing foundations, of course) to under the new garage? Technically it seems possible to me, what I don't see much detail on is what the "roof", or section of basement beneath the garden would look like.

 

How is this typically done? Block and beam on supporting I-Beam steels (the damp proofing detail makes me think this option could problematic) ? A reinforced slab cost in-situ with supporting formwork? I assume whatever the roof structurally looks like, it would be covered in some sort of DPM which makes part of the external damp proofing strategy for the basement? 

[Conor]

Could do with drawings / site photos.

 

Basements are normally built in situ reinforced concrete, either in iCF or temporary formwork. A tanking system is applied. This is brought up 150mm above the surrounding ground level. For the "roof", you'd drop in precast concrete planks, and build your garage as normal on top of this concrete box.

 

What would you be using the basement for? Will it be part of the house? How big? 

[jayc89]

40 minutes ago, Conor said:

Could do with drawings / site photos.

 

Basements are normally built in situ reinforced concrete, either in iCF or temporary formwork. A tanking system is applied. This is brought up 150mm above the surrounding ground level. For the "roof", you'd drop in precast concrete planks, and build your garage as normal on top of this concrete box.

 

What would you be using the basement for? Will it be part of the house? How big? 

 

It would be reasonably sized to connect the house to the garage. Distance between the house and the garage is 16m, to maintain a simple "box" shape to it, width would be approx 12m (because the garage is offset to the garage).

 

If we were to go ahead, it could only be for recreation so cinema room, bar/games room, gym, plant room - we're in a flood risk 3 zone (never actually flooded..), which means we can't use a basement for bedrooms etc AFAIK 

 

 

[Conor]

How much money do you have for this "extra"?  £300k? £500k? £700k? Because those are the kind of numbers you need to have in mind.

[jayc89]

1 hour ago, Conor said:

How much money do you have for this "extra"?  £300k? £500k? £700k? Because those are the kind of numbers you need to have in mind.

 

Even if I do the majority of the work myself? Genuine question as I'm sure there's costs I've not considered, but my, relatively naive, idea was to hire an excavator and dumper, probably 13t excavator and 9t dumper, dig using the "open cut" method (45 degree slope down to the foundations to avoid the need for additional bracing - I think I have the space for that) spread soil out across our paddock (approx 1 acre) so save on grab hires, some sort of passive slab (spec'd by engineer) and ICF walls (again spec'd by engineer), again done myself, and given the "garden" roof, presumably external damp proofing (opposed to internal membrane + sump) + waterproof cement mix as the damp proofing strategy. 

 

The roof was my biggest unknown and where I suspected a large chunk of cost would be hidden, which brings us to this post. 

[Conor]

If you've somewhere for the soil to go, that's a massive chunk saved!

 

It will be a substantial concrete structure. Maybe a 150mm slap with min one layer of mesh, the walls will be 200mm with plenty of rebar. Having part of the garden above will add a certain level of complexity. ICF is an option but tanking will be your biggest challenge. You'll most likely need a third party to do this in order to satisfy building control and house insurers / mortgage companies. It'll need to be a full wrap around as part of it will be under your garden. Other big costs will be drainage, importing a LOT of stone, insurance (excavations over a certain depth will attract a higher premium, for good reasons.)

 

Other than that, basements are easy

 

[jayc89]

1 hour ago, Conor said:

If you've somewhere for the soil to go, that's a massive chunk saved!

 

It will be a substantial concrete structure. Maybe a 150mm slap with min one layer of mesh, the walls will be 200mm with plenty of rebar. Having part of the garden above will add a certain level of complexity. ICF is an option but tanking will be your biggest challenge. You'll most likely need a third party to do this in order to satisfy building control and house insurers / mortgage companies. It'll need to be a full wrap around as part of it will be under your garden. Other big costs will be drainage, importing a LOT of stone, insurance (excavations over a certain depth will attract a higher premium, for good reasons.)

 

Other than that, basements are easy

 

 

Yeah that was along similar lines to what I was thinking. I believe I have the space around the basement area to create a 45 degree slope, although some of that sloped section is where some of the, above ground, extension foundations would go, so presumably I'd need to shutter and pour those prior to backfilling the slope?

 

Grey being the basement outline, brown being the ~3m sloped area around it. 

[Conor]

You'd need to do a soil investigation and have a SE advise. Especially when within 4m of a structure. Our soil was particularly stiff, allowing 60⁰ battering with membrane to reduce erosion.

 

You'll not be able to build standard foundations for the extension on ground that you've disturbed.  Lots of factors and worth a chat with your architect and SE.