Foundations for cavity wall garden room


17 Feb 2022
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United Kingdom
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.


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!

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