Extension/lean-to foundations

[^woody^]

Sorry, whilst the existing base could be with the emphasis placed on (could be) and (presumably) strong enough to support the proposed light weight structure, with out knowing the ground conditions, whether the existing base is the make up you have specified, then in this instance we consider this opinion to be contentious.
Your choice.
Regards oldun

Of course we don't know the ground conditions. In the same way that we don't even know if a standard trench foundation will even be adequate.

But in principle the OP has an existing raft foundation, and a 150mm slab is not going anywhere fast. So on that basis, I can't see a need to install any other foundation or mess around with the ground or an edge beam or suchlike.

Nose has given the standard advice, and you can't argue with that. However, I'm all for building economically and using what is available. And by all accounts there is an existing raft ready to use

Yes, we may all like some new foundations, and masonry walls and everything just like a "proper" extension. But sometimes budgets don't allow, so then its a case of using what is available

 

[noseall]

Yes, we may all like some new foundations, and masonry walls and everything just like a "proper" extension. But sometimes budgets don't allow, so then its a case of using what is available

Fair enough Woods, economy rules but only when the base is not more of a hindrance than a help.

How many times have posters complained about a slab toe allowing seepage through a sectional garage base?

We are not talking raft here but an above ground base.

 

[Deluks]

Surely if the base is lower than the existing internal floor then the 'seepage' problem could easily be avoided by design?

At the end of the day, all the advice being given here can be reasonable argued, the only thing that you should do, whatever you decide to go for, is to keep the existing structure seperated by a weatherproof door.
Which you say you are planning to do anyway. Just don't be tempted to remove it afterwards.