Building new footings next to existing slab and house

[TFP_Byrd]

We are designing a new verandah next to an existing slab on ground and house, but we know nothing about the existing structure. As we need to dig down 1500mm for the new footings, I'm sure we will end up below the foundations of the existing house. I'm concerned about cracking the existing slab on ground (blue) and compromising the footings of the existing house (yellow) in the temporary condition (while the holes for the new concrete footings are dug out). I'm happy to be corrected but I don't think we will have an issue in the permanent condition. I feel it's too much to request the builder underpin the house adjacent to the new footings, but I'd like to know how others here would handle a situation like this? On another note, will we have to design the new footings for surcharge loads from the existing house and slab?

 

[Tomfh]

I would be founding the verandah on the same material as the house, or detailing to allow for differential movement between the two footing systems.

 

[JoshPlumSE]

What about doing drilled piers instead of spread footings? All you'd have to do is know how far out from the house you have to go to avoid the existing footings.

 

[Enhineyero]

if its a 550 diameter footings I don't think it will compromise the existing slab/strip footing, you would need to excavate a long enough strip in order for that to happen. Also, 550 diameter x 1500 deep sounds a bit large for a Verandah.

 

[hokie66]

You will have to do a bit of local excavation to locate the edge of the existing footing. I wouldn't worry about your new footings undermining the house, as they are relatively quite small.

 

[TFP_Byrd]

Thanks for the responses and I generally agree. Should I put some sort of note on the drawing so that I'm no blamed if there is cracking in the existing slab in the future?