New build Basement

[Sjk]

Hello

 

Building hasnt started yet, the idea of a basement is becoming more appealing by the moment.

 

It would not extend beyond the walls of the house, but would give some extra space, and I assume this is the best time to do it.

 

Anyone have any advice? I am worried about leakage but a subcontactor I have spoken to is very confident. I know, how long is a piece of string, how much extra is it likely to add on to the build?

 

Thank you in advance

[Alexphd1]

I have chosen to build a 3 sided basement as it best suites the site. I wouldn't choose to build one if site was flat or many acres and ground around here is relatively cheap. Can't comment on cost as I will be doing it myself. Not to bothered about leakage with being 3 sided design, tanking & will also be using waterproof concrete and everything goes along with that.

[Mr Punter]

A knowledge of the ground conditions is a must.  Also, is the basement far from boundaries, which will make excavation simpler?  What is it to be used for?  Does it need drainage for WC etc?  Any light wells?  If you look at the construction cost per square metre of basement being 1.5 times that for above ground and factor in that the end value will likely be 70% of the above ground, does it look viable?

[Sjk]

I need to get a feasibility study done, if it costs 1.5 times and is worth 70% it will well worth doing.

[ragg987]

All good points from @Mr Punter. We went with an ICF basement (no extra tanking or drainage required) but we checked our water table first, it is below the basement. We kept the area free of water installations to avoid the additional risk and costs, had a single light-well / fire escape on one side and the other side is window-less by design (home-cinema). We were not permitted to connect our surface drainage to the main drains, but we agreed with BC that the light-well drain could be the exception.

 

I would say we were closer to double cost compared to above ground as excavation and muck-away were significant. Hard to be sure our basement costs were part of a lump-sum that included all groundworks, drains, soakaways, cast concrete stairs and beam-and-block cap to the basement. Our site has a compound slope, both back to front and left to right that adds to complexity and cost.

 

An estate agent I spoke to prior to build said sales price of £325 per sq ft for standard accomodation space, but extra spaces (like the basement) was likely to be £180. After build our house was valued at £345 psf on average, so for us it was cost effective and the extra space is great.

Edited May 8, 2018 by ragg987

[Sjk]

thanks for the replies, looks like there is a high water table, likely 3m below ground level.

[ProDave]

3M is a low water table. Here it come to withing a few inches of the surface at times.

[Sjk]

Ha, I guess its relative, the initial ground survey I had done said it was high.

[Sjk]

Just to bump this again.

 

I do need to get a pro out to look into it for me, I was having a difficult time to estimate the costs. I think I can do the excavation myself, while I want to do as much as possible I am worried about compromising the watertightness.

 

The basement would be an extra living/family room and utility area. I wont be able to get it to open out onto the garden, but would have long window overlooking the garden. The basement walls wouldn't back onto the boundaries, and the site is flat.

 

Interesting ragg987, when I spoke to a specialist they recommended using waterproof concrete and a drain/pump system.

 

Thanks again for all the advice, land values are aprx 650£ per sqft, and the basement would be 450sqft.

 

Although it is probably worth it, I cannot justify spending more than 100k, I dont think.. I should be able to estimate the excavation costs now, just need to figure out the rest.