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I'm planning to build a garden office shed about 2.5m wide by 3m long. Regulations in the UK mean I cannot exceed 2.5m high since it will be close to the boundary of my property. The picture below shows a rough idea of the front elevation. I want to use a pitched roof and so it's actually quite tight to make the roof high enough for the door (just over 1.9m).

Garden office front elevation

I've researched insulation regulations (I don't have to stick to these, as it's just a shed, but I do want to make it warm as I will be working there year-round). To achieve the recommended level of insulation, it looks like I would need around 150mm of PIR insulation board. The obvious thing would be to pour concrete foundation blocks and attach a wooden frame to this before inserting the insulation. Something like the picture below:

Shed foundation

The problem is that this would be sitting just above ground level and would thus take away a valuable 150mm from my available headroom. What sort of foundation could I build which would leave the floor level only just above ground level, while also providing decent insulation?

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    Looks like you can exceed the 2.5m limit, that's just the point where you need permission from your local authorities. If you're just exceeding the limits by a few inches, I assume you could get that permission without too much trouble.
    – user3757614
    Commented Dec 20, 2019 at 23:51
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    Insulated Concrete Forms (ICFs) might be something to look into.
    – Charles
    Commented Dec 20, 2019 at 23:57
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    @Charles those look to be more related to walls than foundations unless I'm mistaken?
    – Matthew Dresser
    Commented Dec 21, 2019 at 0:32
  • You can also purchase insulated CMUs (cement blocks) rather than forms.
    – Matthew Gauthier
    Commented Dec 21, 2019 at 14:10

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Change your roof style! It is a shed why have a high pitch? A almost totally flat roof with a membrane surface will do what you want , these hold up well even in costal areas where I live on the other side of the pond, and in some areas even a 1/2 inch two tall ends up in legal issues , a almost flat membrane was the way to do it , it holds up well drains even heavy rainfalls and they can last a long time if properly installed, want to go a bit cheaper rolled roofing that is heat sealed , not as good as membrane but much cheaper. I agree icf are blocks for walls not foundations, high density foam under a slab is a cost effective way to insulate and add a hydronic heating system and it could be quite nice.

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  • Thanks for the answer. I wanted to go with a pitched roof to be more in keeping with the style of the house and neighbouring houses.
    – Matthew Dresser
    Commented Dec 21, 2019 at 11:45

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