Spotted a small plot with full planning permission. Adding a basement?

[flanagaj]

I have identified a small plot which has planning for a 3 bedroom detached house.  It's quite small and there isn't really scope to increase the footprint of the property as the original plans have already been slightly increased.   So unless there are services running underneath the property which would make a basement tricky, would adding a basement be a possibility so as to add the required extra space?

[Thorfun]

a basement 'shouldn't' affect planning even though the local council may try. our local planning office suggested that our total living space was disproportionate to the existing building and we should get rid of the basement. when we pointed out that the basement isn't visible so how could it possible be taken in to consideration they backed down.

 

a few others on here have had that experience as well.

 

But....there's never any guarantees when it comes to dealing with the planning office.

[Bitpipe]

Yes - exactly what we did.

 

If there are services in the way then they'll be in the way for your foundations.

 

If the basement is full footprint then it acts as the foundation of the house so is cost effective.

 

We applied for planning for the above ground house first and then added the basement on a second application, sailed through.

[Ferdinand]

Be aware how close you are to the boundaries of your plot and neighbouring buildings; potential complication with eg Party Walls // Party Wall-Fences, and perhaps access to the rest of the plot if the basement dig takes up most of the width.

 

[Scoobyrex]

and look into the cost of a basement,  my neighbor was quoted an additional 200k on his build for a basement in a 4 bedroom house.

[Bitpipe]

21 minutes ago, Scoobyrex said:

and look into the cost of a basement,  my neighbor was quoted an additional 200k on his build for a basement in a 4 bedroom house.

 

By who? What were his ground conditions & site conditions? These are the two biggest variables.

 

We built a 120m2 basement to a passive standard for £100k and given we saved on traditional foundations / slab the incremental cost was about 60%. This was in SE England in 2015 so I appreciate that prices may have gone up.

 

The fit & finish happened at the same time as the rest of the house so extra electrics, plastering, joinery, decorating  & flooring but it was not noticeably driving up the cost of the build - economies of scale and all that.

[Scoobyrex]

He is using independent ground workers, builder for the shell and individual trades for the interior.

 

Sorry do not have further details,  he told me the price was higher than anticipated and has abandoned the idea. 

[Bitpipe]

7 minutes ago, Scoobyrex said:

He is using independent ground workers, builder for the shell and individual trades for the interior.

 

Sorry do not have further details,  he told me the price was higher than anticipated and has abandoned the idea. 

 

Independent ground workers are the way to go, I thought for that price it was a 'basement company' who normally charge a premium.

 

I wonder if he had the ground conditions already determined and a SE approved plan or if it was a budgetary 'worst case' figure.

 

Until you've spent some money on the investigation, you really can't design (and therefore cost) the basement with any confidence and need to accept that you may find conditions that go beyond what your budget allows. However the report can still be useful for foundation design so not totally wasted money.

 

Site conditions can also determine costs, especially if things are very tight wrt neighbours and access. Sheet piling may be required if you don't have the space or soil type to batter back the excavation and the party wall rule starts from the edge of the hole, not basement wall.

[puntloos]

On 24/06/2021 at 11:56, Bitpipe said:

 

By who? What were his ground conditions & site conditions? These are the two biggest variables.

 

We built a 120m2 basement to a passive standard for £100k and given we saved on traditional foundations / slab the incremental cost was about 60%. This was in SE England in 2015 so I appreciate that prices may have gone up.

Can you elaborate on this somewhat? 

 

The crucial question I have is, how much is a 100sqm traditional slab, vs how much is a 100sqm slab that-so-happens to have a basement embedded in it.

 

Basically the "net cost" of adding a basement...

 

My house is already way bigger than I wanted or need (literally 3x floorspace of my current house) but there is a certain level of elegance to having all plant electrics, networking etc in there, so I'm really only looking at a 25sqm, not full-slab basement.. 

 

I'll have soil investigation back soon but the SE said it looked fine, standard british clay, not a good soakaway though. 

 

 

 

[Bitpipe]

3 hours ago, puntloos said:

Can you elaborate on this somewhat? 

 

The crucial question I have is, how much is a 100sqm traditional slab, vs how much is a 100sqm slab that-so-happens to have a basement embedded in it.

 

Basically the "net cost" of adding a basement...

 

Obviously it depends massively on the ground & site conditions and design and you need to spend some money to get a definitive answer,  Your GI survey and SE fees for a basement will be more comprehensive than for just a slab or foundations - you need to get this far to get sensible quotes and make a true comparison.

 

But let's make some simplistic assumptions.

 

If you were planning a 10m x 10m passive raft foundation, 200mm thick sitting on 200mm EPS, then structurally you're adding the walls (40m x 3m x 200mm plus 200mm EPS to exterior) plus the ground floor, which can be suspended timber floor or a concrete lid - we went for the former and it has worked well with UFH in spreader plates. Needed some steel support to bridge the gap and take point loads from the timber frame above.

 

You need to excavate and remove the spoil (11m x 11m x 3.5m) and allow for backfill of the working space. Muck away is volume based and different materials bulk up (chalk is 3x, clay 2x and stone / gravel is 1x). If your spoil is 'clean (WAC test required) then all good, any contamination will jack up disposal costs significantly).

 

Waterproofing per site conditions & SE design.

 

Internally you need to frame out the room spaces and meet regs wrt fire  - either an Independent exit to ground level or sprinkler system.

 

Add electrics, & finishes (plastering, joinery, decoration).

 

So, our basement structural construction package was £120k but that included demolition of existing house (£5k) and all drainage & services (£15k) so call it £100k for the basement structure itself. The quotes we got for traditional strip foundations (we had clay on gravel on chalk so assumption was going down about 2m) were about £40k and a passive slab was about £28k but that was excluding prep ground works which I expect would have added another £10-15k.

 

Framing out the rooms was about £1500 and the additional doors / plasterboard / electrics etc were not excessive as they were quoted as part off the whole house fit out.

 

Now - we were lucky to have a large accessible site that could accommodate a lot of plant and did not need to worry about party wall wrt the excavation. No meaningful ground water so just waterproof concrete and land drainage were sufficient. 

 

If you're on a tight site and need sheet piling plus a more comprehensive waterproofing solution (and worst case, de-watering during the build) or on poor conditions (e.g. made ground) needing piling etc then your construction costs would be significantly higher.

 

 

 

 

[puntloos]

17 hours ago, Bitpipe said:

 

Obviously it depends massively on the ground & site conditions and design and you need to spend some money to get a definitive answer,  Your GI survey and SE fees for a basement will be more comprehensive than for just a slab or foundations - you need to get this far to get sensible quotes and make a true comparison.

 

But let's make some simplistic assumptions.

 

If you were planning a 10m x 10m passive raft foundation, 200mm thick sitting on 200mm EPS, then structurally you're adding the walls (40m x 3m x 200mm plus 200mm EPS to exterior) plus the ground floor, which can be suspended timber floor or a concrete lid - we went for the former and it has worked well with UFH in spreader plates. Needed some steel support to bridge the gap and take point loads from the timber frame above.

 

You need to excavate and remove the spoil (11m x 11m x 3.5m) and allow for backfill of the working space. Muck away is volume based and different materials bulk up (chalk is 3x, clay 2x and stone / gravel is 1x). If your spoil is 'clean (WAC test required) then all good, any contamination will jack up disposal costs significantly).

 

Waterproofing per site conditions & SE design.

 

Internally you need to frame out the room spaces and meet regs wrt fire  - either an Independent exit to ground level or sprinkler system.

 

Add electrics, & finishes (plastering, joinery, decoration).

 

So, our basement structural construction package was £120k but that included demolition of existing house (£5k) and all drainage & services (£15k) so call it £100k for the basement structure itself. The quotes we got for traditional strip foundations (we had clay on gravel on chalk so assumption was going down about 2m) were about £40k and a passive slab was about £28k but that was excluding prep ground works which I expect would have added another £10-15k.

 

So to be clear:

 

No Basement:

40K - Foundation

28K - Slab

5K - Demo

15K - Drainage

--- 88K - Total

 

Basement:

100K - structure

5K - demo

15K - drainage

--- 120K sub total

and then dressing the basement: (electrics, finishes, sprinklers, walls, stairs) - maybe 25K?

 

So effectively you are paying 32K extra for the raw basement, maybe 57K extra when all is said and done?

 

 

[Bitpipe]

11 hours ago, puntloos said:

 

So to be clear:

 

No Basement:

40K - Foundation

28K - Slab

5K - Demo

15K - Drainage

--- 88K - Total

 

Basement:

100K - structure

5K - demo

15K - drainage

--- 120K sub total

and then dressing the basement: (electrics, finishes, sprinklers, walls, stairs) - maybe 25K?

 

So effectively you are paying 32K extra for the raw basement, maybe 57K extra when all is said and done?

 

 

 

Hmm, I would have had either slab or foundations, probably not both but the slab would have cost more as that was just the build price and did not cover prep.

 

I'd say the basement was £50k over for structure and £15k to finish, it really did not need a lot of work.

 

But again, my site and ground conditions were very favourable and that is the big variable, the latter needing some investment (GIS) to confirm before design and accurate costing can be completed. BGS data can give you a good estimate if you can find a borehole in the vicinity.