Generally speaking a new 40 square (370M²) double storey house will be less expensive to build than a new 40 square single storey house. This is mainly because a two storey house will have a smaller “footprint” or slab area and a smaller roof area.
While the two storey house can also be built on a smaller less expensive block of land and can allow for more outdoor and garden area the need for a staircase or elevator can add additional building costs to the project.
Single storey houses are still very popular with mature owners who often prefer to live on a single level home that is more conducive to having a garden outlook from almost every room.
Due to privacy regulations “overlooking” into neighbour’s properties when designing a new two storey house attracts several restrictions on the size and placement of upstairs windows and floor plan boundary set backs which can have a significant effect on the Architecture.
Overall a double storey house will be a more economical option but ultimately the decision will come down to your personal preference and stage of life.