Foundations for chalet bungalow extension

[olliesquire]

Hi
We have an existing single storey side extension on our chalet bungalow.
We are hoping to make this existing single storey extension into two storey by building a ‘chalet roof’ on top with dormers - thus creating rooms in the roof (in other words as per the rest of the house).
In theory, we wouldn’t need to redo or add to any of the existing brick walls, but just build the roof upwards from the top of the brickwork….but of course this is assuming that the existing foundations are OK...
With respect to the foundations, the plans I have seen for the single story extension specified foundations “….min 1000mm deep and laid with GEN 3 concrete….”.
However, I spoke to building control today, and they said that in the approved docs, the foundation is “850mm deep into stone” and was approved by BC for the existing single storey extension.
Presumably because the foundation is ‘into stone’, they can be a little less deep?? And presumably this would also indicate good strong foundations..?
They suggested that we dig a trial hole to expose the footings for a BC inspector to inspect to give a view on their suitability – which we will do.
However in the meantime (since we are needing to get a budget together for this asap), what is anyone’s gut feeling as to whether a foundation ‘850mm deep into stone’ might be suitable for adding a chalet roof and dormers. Again bearing in mind that it would appear to me to be just a roof and no additional brickwork!
Or is this not enough info for even a gut feeling…?
Hope someone can help…

 

[RonnyRaygun]

Depth isn't governed by the load onto the foundation, it's governed by ground conditions.
If you have undisturbed stone 850mm below ground level then that will be fine as far as depth goes.
If you have a soil type that is not influenced by tree roots or affected by frost action, then you don't need to go any deeper than 450mm.
Of course it may be that you need to dig to 850mm as you have a different overlaying soil type that could be influenced by trees or frost, and you need to dig to a suitable bearing strata.

In short, there shouldn't be anything to stop you using the existing foundations as long as they are wide enough. Width is the most important factor when adding extra load, but I would be pretty confident that if you are bearing onto stone the current foundations will be wide enough for two storeys.