Extending concrete base to accommodate a SIP Garden Office

7 Oct 2008
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United Kingdom
Grateful for some advice in getting my project of the ground as it were!!

I have decided to take on a project and build a 7mx5m garden building to be used as a den for the children. The full 'turnkey' estimates I was getting were in excess of £20k. I have found a company that can provide the flat pack building for approx. £8.5k and I will need to provide windows/doors, cladding, roofing.

To commence the works I need to ensure I have the right foundations. There is a concrete base 10mx4m already in place so am looking to extend this by 1.5 - 2.0m. I have started digging to see how deep the existing base is - 300mm, and have continued at the depth.

Do I continue at this depth and look to put some hardcore in the bottom and get the cement pumped in (I have no real access) and levelled to the existing base? Do I need something to 'join' the bases together?

There is an option to have SIP panels to insulate the floor. Is someone able to advise how best to achieve this? Lay some DPC, build a timber sub frame, lay the panels and then cement over the top or just lay the building floor over the top?

I am looking the forward to this project but have not tried anything like this before :confused:
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It may be best to ask the company that are supplying the building what the requirements are, no doubt they will have a minimum specification that you need to comply with in order to maintain the buildings warranty / guarantee, then once you know what they recommend come back on here and ask what you need to do to achieve it.
Thank you for your reply. The company do just manufacture the panels and therefore cannot give guidance on foundations although have mentioned that a foundation plinth system with timber subdeck will be sufficient.

As the majority of the area is already concrete base I thought I would continue. I am also hoping that by going this route I would reduce the instance of vermin getting underneath the building. It would be useful to hear from other members that have undertaken this kind of project so perhaps they can make me aware of any pitfalls!!
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Further to my earlier post I thought I would attach some photos of the site which show the existing base and what I have dug out. I am hoping that someone will be able to advise on the following: -
I) Whether I should add some hardcore to what I have dug out before covering with cement
ii) Should I just fill the additional space to the level of the existing base or cover the entire area with an additional layer and if so how much?

My other questions relate to the sub deck of what the Garden Office will be built on. I was looking to lay DPM on top of the base and then lay the timber sub deck with insulation in between.

Would a perimeter of bricks, including air bricks be best to install the deck on?

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