Concrete slab

[sunburn]

Good morning
I am planning to build an 8 meter by 4 meter, 12 foot high office in my garden with concrete blocks.
Currently there is a 12 inch concrete base covering a 1/3rd of the area where two sheds used to stand. I have dug out the necessary soil for the rest of the area. Could someone please advise me if it is ok to board and peg out the rest of the area, fill it with concrete, bringing it up to the 12 inch level and use this as a base.

Thanks in advance

Sunburn

 

[Deluks]

Are you sure it's 12" thick all the way through, and not just at the edges? It's very deep for a garden structure, you could park a tank on 12" of 'crete! Not to mention the cost and work involved in laying it.

If it really is 12" all the way through then this will be fine to build off of.

For the new section, it will be more economical to go deeper at the edges where the walls will sit, but the central floor section can be 4" deep (concrete mesh recommended) As you are tying into existing, insulation should go on top of the lot (as opposed to underneath the new slab)

Where the slabs meet, a step should be cut into the old one, and a layer of polythene separating the two.

You should shutter and pour the edges first, this will give you an 'enclosure' for the hardcore before laying the floor slab.

 

[sunburn]

Deluks, thank you for your reply.
Your method will undoubtedly save me money and effort
Trouble is now you've got me thinking!!
Apologies if this sounds obvious to you.
As I've already leveled the ground to the bottom of the existing 12" base.
Should I make a channel for the sides take those up to 12", wait for that to set and fill the center with hardcore, reinforcing steels and 4" concrete?
I've been advised that the footing should be 4" wider than the block on both sides. Does that seem reasonable?

Once again thank you

Sunburn

 

[Deluks]

Yes, you seem to have your head round it. The question that needs to be answered is how deep to dig the perimeter foundation. 12" probably won't suffice. Standard procedure is to go down at least a metre, lay a concrete strip or full fill with concrete, and start to build off of that. As previously mentioned, soil type and vegetation can affect these figures wildly.

How are you planning on building the office, dense concrete block? lightweight block? brick? cavity wall? piers? single skin? flat roof? pitched roof? internal supporting walls?

More importantly, how long do you want it to last?

Got/need planning permission?

Apologies, if my encouraging you to think has troubled you so.

 

[sunburn]

Deluks, good morning
Many thanks for your time to help me understand this, more importantly for me not to attempt a build and have it collapse and kill me!!
Following your question is the existing base 12" lead me to take a sledge hammer to the side of it to check. Your insight was correct, it's not solid, but what looks like a 5 course retaining brick wall, hardcore center and 3" concrete lid.I'm not sure if the edges are strong enough to hold the weight. I get the feeling I'm going to have to demolish the lot.
If that the case I will be down to a flat, level earth. I was hoping not to have to dig any more, as I already went down 12" to get to the bottom of the existing base. Your suggestion of going down another meter I assume is the safest and cheaper way. What are your thoughts on a slab from where I am, at ground level?
Just to make things clear, I'm planning to use dense concrete blocks,single skin, rendered and have a tiled pitched roof.

No apology need for getting me to think. I'd rather think than have 14 tones of expensive concrete sat at the end of the garden!!