New house being built, wanted to get a Range but electrician says house is only 63 amps

Not an electrician.

So your MIC will be 12kVA (can confirm with ESB) which will give you an 80A main ESB fuse and that usually means a 63A cut-off. The tails will be 16mm^2.

To get an enhanced supply (16kVa), you need meter tails of 25mm^2 which will give you a 100A ESB fuse and an 80A cut-off fuse. You will also need a fuse-board rated 80-100A. In all likelihood, the cables go to your meter are already sufficient.

In all honesty, a new build with a 63A consumer unit is pretty crappy in this day and age. If I was getting a new build, I would be looking for a 16kVa supply at a minimum.

I just got my supply upgraded to 12kVa from 8kVa in a late 70s build and no work was needed from ESB.

Couple of options I can think of

You can pay ESB Networks to upgrade your supply to 16kVA, this will require a new cable from the meter to the CU and an upgraded fuse in the CU

Another option is a soft starter for your heat pump (I assume that's what you meant by heat exchanger). This device basically slows down the startup of the heat pump so it doesn't produce a large current spike. You may still face a perfect storm if the hob, oven and heat pump all kick on at the same time

Another possibility could be a priority switch between the hob and heat pump. This basically stops one from switching on when the other is on. Probably a bad idea though since your heat pump or hob will lose power which they don't like doing

Its worth investigating if your heat pump has any ability to monitor the power load of the house and self limit. You'll likely want to talk to the manufacturer about this

A final and simpler workaround is to just turn the heating off when cooking. Cheapest solution, but much more hassle