Build extension around existing smaller extension before knocking it down?

[HammerHappy]

I would like to build my 2 storey extension around the existing single story extension so we can carry on using it (part of kitchen) then once new extension is water tight, knock the old one down.

I understand this is probably unusual and it no doubt would be easier to knock it down first, but is it possible?

The main 2 problems I can think of is :

Existing ext will be in the way when using a laser level to set out foundation levels.

Won't be able to put ground floor in. Not sure yet on the construction of the floor, most probably suspended wood as it's a sloping site with a depth of around 800mm from ground to floor level at its deepest part.

If I chose block and beam floor could that be fitted after extension is built? Obviously if the beams have to sit on inner leaf then this isn't possible.

I am building the extension which will have kitchen diner on the ground floor and 2 bedrooms one with ensuite on the first floor, using standard concrete block cavity wall construction and building control will be following me all the way.

I would really appreciate any constructive viewpoints and ideas.

Thanks

 

[John D v2.0]

Well, we did a side and rear extension followed by knocking through on both existing walls. You are just proposing the same thing only on 3 walls instead of 2. I don't remember any specific problems with knocking through after, they just knocked through completely before starting to make the new floor (suspended timber), then they lined it up with the old one. But building a new one would have been as easy.
I think the main problem would be extra cost of working around existing walls, but I'm not sure the specifics.
The floor is basically free standing not relying on the inner leaf or anything.

 

[^woody^]

None of those problems are actually problems.

Particularly in context of the benefits.

 

[noseall]

We have done something similar and its not that unusual. Measuring is likely to be a pain particularly when trying to square up. As long as there are no drain runs etc, that need incorporating, then crack on.