How much to extend and retrofit?

BarraOG

We live in a typical four-bed two-story estate house in Kildare, built in the nineties. We are exploring the idea of extending since we both work from home and the kids are fed up of us using their rooms as offices. We're also considering buying instead.

I contacted the council and they were very positive about our chances of extending. A house in our estate was extended a few years back but I havent been able to make contact with the owner. I have looked at their plans, which extended from 120m2 to 175m2, adding a bedroom and office. This extension moved an external wall to make the house wider and then extended at the back too. These options are also available to us. The blue shows the extension from the front.

The stairs had to be redone and it looks like the hot press might have been altered / moved. Downstairs bathroom was also altered since the room was widened. In our case, such an extension would effect the location of the kerosene boiler too. So a good bit of plumbing is required.

If adding such an extension, how much retrofitting would make sense to do at the same time? I'm thinking in terms of windows (for the sake of consistency), ground floor insulation, and preparing for the possibility of someday installing a heat pump.

Pumping the walls and better insulating the existing attic can be done at anytime I presume.

Would a good reputable builder been able to adequately retrofit or is it more of a specialised job?

So we are trying to get a very rough idea as to what it would cost to get a similar extension, as well as ground floor insulating the entire house and replacing all the windows and doors. I'll lay the new floors myself and do the painting, skirting and architraves. Have a friend who can help me hang the doors. So a builders finish is what we are after.

Any other advice? Thank you!