Attic conversion vs air tightness

First off a big hello to everyone. First time poster long time lurker

We are looking to get some opinions from the experts on the board regarding attic conversion in an air thigh house.

Our house is built using single hollow cement block capped on the outside with 150mm insulation. The house is airtight with mechanical extract ventilation.
For those of you interested in details you can find a nice blog below which describes in more detail how the house was built.

passivehouseplus.ie/magazine/new-build/ground-breaking-housing-scheme-captures-one-developer-s-journey-to-passive

We would like to increase the house space by converting the attic and creating another bedroom and a small en-suite. I do worry about breaking the membrane that was laid over the whole house but I said I'll ask the pro's and send a few emails around for quotes.
The first builder to respond asked a bunch of questions about the house was built etc. When finding out the house is air tight he said not to bother with the conversion as breaking the membrane will loose the BER certificate ratting and cause issues when selling the house. He also said the attic can be converted and preserve the air tightness but it will cost significantly more.

I would like to hear other opinions from people on here. Is this really not doable and we should forget about it? The other option is to build a garden room which I think will be in around the same money.

Thanks,
Julian