Hi ,

If you having any any records how your extension has being build previously will be helpful! Specially if you are planing to add a brick work on top of your existing walls foundations needs to be checked if are suitable enough for another room including the roof weight
This can be find out digging a trench hole from outside building and find out the depths of the foundation’s, 600mm deep foundation will be good enough in your chase

Also you need to consider existing timber roof structure ! Using your existing flat roof as a floor for the new bedroom this floor joints needs to be at least 175mm x 50 mm

Regards
Space Above Ltd

Good answers here.
If there isn't 600mm foundations then a structural engineer can advise.
Alternative cost effective options are timber frame above or and ive seen this done and LABC have passed it is thermo block.
Always communicate with LABC, Architect, structural engineer.

You would need to contact building control if the single story extension was threw building control they will have paperwork on the extension and will no way concrete went into them if the single story extension is not threw building control they will be lookin it took down or will ask for a hole dug out to see were old rounds are to

Dig a inspection hole and ideally it wants to be 1m below DPC to bottom of foundation depending on ground build up.
New floor hoist sizes will depend on span size of building

Hi have you a completion certificate for existing extension if so would be straightforward if not a test hole would need to be dug with a 1 meter minimum foundation subject to local building control if there is trees nearby could want more