Thanks for the replies guys,
I am not sure that chalet is the best name but I see that this is how these types of houses are advertised as. I uploaded a picture with an example. These are semi-detached houses that originally had a slopping roof and it is very common to square that and create an extra bedroom and a bathroom upstairs. You can easily see in that picture where the extension was added to the house.
To give you a little bit of context, we already have an offer accepted on an extended house like the one in the picture that we like very much. However, since the covid lock down, our vendors told us that they cannot look for a house to move so we are in waiting now. However, this crisis might change things, they might not want to sell anymore, house prices might go down etc. So we were thinking maybe we could buy a cheaper house that is not extended and look around for some builders and architects to get some quotes on how much a job like that would cost. The area were we want to buy is full of this type of houses and I understand from realtors that permission planning won't be an issue.
I am interested if anyone had a similar job done and how much work is involved, ballpark costs and other experiences.