Foundation type and effort estimate 8m x 4.8m cavity wall outbuilding

OJ1

Hi Everyone – first post here. Been looking at your forum for a while now and its given me lots of really useful advice.

 

The specific advice I am after here is around the footings and oversite element of the proposed build below.

 

Situation

 

I’ve gained planning permission to build a 8m x 4.8m block cavity wall, single storey structure at the bottom of my garden ( the distance between the structure location and the roadside is 60m, the side access allows for the 1 ton dumper and a mini digger). The structure is located far enough away from any other structures to not need the Party Wall Act. I’d also consulted local Building Control who confirmed my plans fell outside of their scope.

 

Due to the positioning of a house to the rear of my boundary, the planners wouldn’t allow me to go above 2.5m from ground level for the building height. As id like to use exercise equipment in there I need a 2.4m internal ceiling height. Given the span of the walls, and getting some decent insulation in the roof structure im looking at around 300mm for the roof itself (390mm when taking in to account an adequate fall).

 

To achieve this ill need to have the internal floor of the structure around 25cm below ground level (not ideal I know). To get this to work ill be looking at installing a soakaway along with French drains – and the ground to the front of the building would also be excavated and terraced (with the back and side walls tanked appropriately to allow them to sit below ground level).

 

The location of the build is reasonably level ground which has been largely undisturbed for atleast 40 years according to my neighbour who has lived there a long time. Having installed concrete fence posts next to the site location and going down approx.. 3ft, the soil is very firm and didn’t appear to have any clay in it.

 

I have very basic DIY skills, this forum has helped me fit a kitchen and a bathroom when COVID originally struck and I was left with a partly refurbished house and no one to finish it off! – but other than moving materials around on site, I hope to use 2 individuals who helped with a previous project. I intend to buy materials and organise items such as machinery hire, which I did manage to do reasonably successfully when we first bought this property.

 

I understand that the footings and getting up to DPC level can often be the most unpredictable element of a build. I am keen where possible to minimise the number of concrete deliveries (and pumps which for 60m is coming in very expensive).

 

One option I had come across was a raft foundation, which may reduce the number of concrete deliveries. Based on research so far :

 

30m3 of earth excavated ( I have space to store this around 50m from the building location, within the distance a grab lorry can pick it up)

200m thick slab with 8mm rebar placed at 200mm centres in a grid formation

On the perimeter of the build at a width of 500mm the concrete would go down to 500mm with rebar placed within it.

Underneath the slab there would be a membrane which would be brought up and placed against the inside of the external cavity wall leaf above ground level.

150mm of hardcore compacted on top of the soil.

 

I will have access to a  1.5t Mini Digger as well as a 1t dumper and a relatively straight 50m to dump soil.

 

Questions :

 

Would the Raft foundation above likely be sufficient for what I want to achieve?

Based on the location of the structure, individuals and machinery available….. how many days should I account for to get 1) the excavation and spoil removal 2) the area sufficiently prepared for a concreate pour and 3) the pour itself?

How many days would I need to allow for between the concrete being poured and starting to lay the blocks?

 

Hopefully I’ve given enough information, please let me know if I have missed out anything important. The individuals I hope to use wont be available for a few months, so any advice on the above would help greatly in me estimating how much cash to set aside/save!

 

Cheers

stuart45

If you can get a dumper through, can't you use that instead of the pump?

OJ1

stuart45 said:

If you can get a dumper through, can't you use that instead of the pump?

Hi, thank you for your reply.

I could do that. I was unsure though as to whether a 1ton dumper and the 60m run would be able to shift 12m3 of concrete in the time the concrete companies locally allocate (spoke to 2 different companies who said they would allocate 60mins with every extra 15mins charged at £35 [ you could argue though that even taking 1 extra hour would only be about a 3rd of the price of the pump!!)??

Do you or other forum members have a view as to whether my proposed foundation solution would be appropriate, and a view on effort estimates for each part of the job? do the below feel reasonable : 

Based on 2 individuals and the machinery previously mentioned
-  Mark out the site, Excavate and transport 30m3 of soil to the collection point 50m away - 2days
-  Create shuttering, apply Hardcore to the area and compact, lay the rebar and membrane ready for pouring - 2days 
- Pour concrete, tamp - 1 days

Lastly, how long would i be required to wait for the concrete to be of sufficient strength to start the block work (2 days?)

Thanks again for you reply 

Section62

OJ1 said:

I’ve gained planning permission to build a 8m x 4.8m block cavity wall, single storey structure at the bottom of my garden ( the distance between the structure location and the roadside is 60m, the side access allows for the 1 ton dumper and a mini digger). The structure is located far enough away from any other structures to not need the Party Wall Act. I’d also consulted local Building Control who confirmed my plans fell outside of their scope.

If you got planning consent for the building then you need to build what is shown on the plans you submitted, or otherwise get confirmation (in writing) from the planners that any changed details are acceptable to them.  Lowering the level of the floor is potentially a significant change to the design - don't assume the planners won't care about that - even though a small change won't be an issue.

Lowering the floor below ground level has significant design implications - you really need some expert advice on this to make sure you get it right.

OJ1 said:

Would the Raft foundation above likely be sufficient for what I want to achieve?

You need to employ a structural engineer or other expert to review your design.  Helpful people on the internet can't really do what you need done.

To save money (e.g. on concrete delivery/placement costs) you are seeking a 'what can I get away with' approach to the design.  This is usually a mistake - if you scrimp on the design (particularly foundations) you can end up paying a lot more later to fix problems.

FaceHead

This whole business of lowering the floor seems like a lot of effort and cost, and never ending damp issues. 

If you can't have the height were you want it, can the structure be moved away from the boundary? How close is it to the boundary? I'd expect if over 2m from the boundary then it could exceed 2.5m.

Doozergirl

I'd look at building a SIPs building instead and using some heavy duty ground screws.  

OJ1

Section62 said:

If you got planning consent for the building then you need to build what is shown on the plans you submitted, or otherwise get confirmation (in writing) from the planners that any changed details are acceptable to them.  Lowering the level of the floor is potentially a significant change to the design - don't assume the planners won't care about that - even though a small change won't be an issue.

Lowering the floor below ground level has significant design implications - you really need some expert advice on this to make sure you get it right.

Thank you for the reply. Yes i think i probably do need to get some expert advice......the planning was awarded several months ago, but its only now where ive got the time to start thinking about progressing it. Now sure how much an engineer might cost do design something like this?!ill have to make a few calls.

I did discuss with both planning and building control my intentions and no concerns were raised or additional information asked for. Originally i had looked to do a structure within permitted development - however when i realised i would likely need to get an LDC at the end i thought it may just be better to get the permission up front.

While going through the process i realised the limitations of a 2.5m structure in internal headroom and originally applied for a 2.85m structure. This was advised as being unacceptable due to the location of a property toward the rear of my boundary. I then amended the plans and shared with planning and building control what i intended to do (this included the materials used for the walls, roof, and also my intention to maximise the internal head room by going below ground level). No concerns were raised or more information asked for before the application was approved.

When i first got the approval i did contact a local builder who suggested a 600x1000mm trench, but as i said i havent had the time to progress things any further. Even getting a rough estimate of what to expect to pay to get to the point where blocks could be laid would be really useful.

FaceHead said:

This whole business of lowering the floor seems like a lot of effort and cost, and never ending damp issues. 

If you can't have the height were you want it, can the structure be moved away from the boundary? How close is it to the boundary? I'd expect if over 2m from the boundary then it could exceed 2.5m.

Thanks, yes i agree it definitely makes things more complicated. When i spoke to planning they advised that could be an option, and then build it via permitted development. Although i technically have the space to do it, it would mean rotating the structure 90 degrees (its currently 1.5m from rear boundary, 1m from each side boundary and taking up alot more space in the garden (which would mean clearing a few trees in the process!)

Doozergirl said:

I'd look at building a SIPs building instead and using some heavy duty ground screws.  

Thanks, i had originally looked at these panels when i was trying to get an idea of what the structure could be made from. Id have to have a proper look but i didnt think these would address the internal height issue with the external height limit i have, and would have thought sticking them below ground level might not go so well!?

As is probably plainly obvious, im very much at the start of trying to organise this build - so thank you all so far for your comments.

stuart45

Are you doing the blockwork youself?

OJ1

stuart45 said:

Are you doing the blockwork youself?

No, unfortunately my abilities dont extend to laying blocks. On this project ill be limited to moving materials around!

 I have a few contacts i could use for the walls. Its the groundworks element im stuck on trying to get an indicative cost for, to decide whether or not suggestions as per other forum members' comments might be something to consider (alternative construction methods or building above ground level).

based on the existing footprint, if i were to end up doing trench foundations 600x1000mm - what would be a reasonable range cost wise to expect to pay to get to the point where i could start the blockwork? am i right in thinking that the ground works is the most difficult part of a project to provide cost ranges for?

cheers

Doozergirl

I was thinking that you'd landscape appropriately to keep the ground level down around it.  The SIPs suggestion was mainly to move away from the concreting issue and make it an easier overall build.  

OJ1

Doozergirl said:

I was thinking that you'd landscape appropriately to keep the ground level down around it.  The SIPs suggestion was mainly to move away from the concreting issue and make it an easier overall build.  

I hadn’t thought about that approach , thanks I’ll have to give it some thought. 

I also wonder if your suggestion of landscaping ‘around’ the structure could also make it less risky to build if I stuck with the idea of a block build. If I were to lower a larger section of ground surrounding all 4 sides and provide appropriate drainage - would that go some way towards building it at ground level ?

thanks I’ll have a think !