The fire is in & has been lit to see if it’s all OK and also to help with any last drying out of the inside. Being mostly timber frame construction this is a lot less than a concrete building requires.
The slate slab under the fire is the first bit of final floor to be in the house.
Even with doors being left open the house was rapidly super warm with the fire. I know it’s not a cold time of year, but it does show how a well insulated and sealed house is easy to heat up and then stays warm.