That is absolutely beautiful missfliss and lovely to see it finished.
Welcome iggety. Your post struck a chord with me becuase our builders originally said 12 weeks. They started in february and hopefully we should be finished in october!
Ours is 2 storey and I think covid delayed it a couple of months but I think 12 weeks is still pretty optimistic. One thing to check is whether they subcontract any of the work. Now we are on the final stages they are subcontracting electrics, plumbing and plastering and I am arranging tiling and decorating. This is fine except that trades are all exceptionally busy at the moment (everyone is doing up their home after lockdown) and there is often a few days wait for people which adds significantly to the timeline.
The other thing which adds is waiting for floor screed to dry - we were told 6 weeks before tiling. We are still having supply issues with windows, there are some we have been waiting for for 4 months. Our builders stockpiled plaster, I am not sure how the supply of this is now. Also if you are importing anything I would make sure you get it before january due to brexit delays and tariffs.
Sorry to be so negative, it probably doesn't help that we are in the middle of it at the moment, others might have a more positive perspective, or it might be different in other parts of the uk. We are in the SE.