Connecting old footing to new footing for single storey rear extension

Standard practice to join footings with bar. The only way I can see you doing it is to break out some concrete drill and resin the bars in and repour the concrete.

It's standard practice when pouring new footings as an extension to drill into the existing footing and fix rebar so that when the new footings are poured they tie into the existing footings. Building control should have made sure this was known but to be honest your builder should have known this. I would say you'll have to cut/break out the newly poured concrete 1metre back from where it joins the existing footing drill and fix rebar to both old and new footings and pour concrete, however run it by building control first to keep them in the loop.

They should have known and done it but it's not the biggest issue in the world to solve. I assume that the size and spacing of the dowels has been specified, if not, then just ask, they will be happy to let you know. Depending on the thickness of the original concrete, the dowels want to be at least 3" - 4" deep from the top. To put them in now, use an angle grinder and breaker to dig out the relevant amount of floor and then concrete in the dowel. In effect, you will be dove tailing the two floors together to look something like this. https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcR8c7vKeR0NkcQrCTh44NBgWHum7GS8_QlfgA&usqp=CAU

Years ago we never put re bar into existing foundation so I wouldn't worry about it

I have worked in quite a few extensions as a labourer, none have needed the bar, it might of been the structural engineer being keen