We have a property on the shores of Iskar Lake, where we are considering replacing the existing summer house with some sort of permanent building.
In discussing the possibilities with an an architect, she has advised us that if we build a wooden cabin type house then a geological survey will probably not be necessary. But if we decide to go for a conventional build ('massive buld' is I believe the Bulgarian term this type of structure)', then to satisfy the planning people, we will definitely require one to be carried out. This will be in the form of an initial viewing by a geologist, who will then decide if a full survey using ultra-sonic equipment will be necessary. The architect actually telephoned a specialist company whilst I was with her to ask the costs and was told that if it was the 'full monty' it would be in the region of 1,200 euros.
However, this is only necessary because our property on the lake shore is in a particularly sensitive area. Under normal circumstances, it may well not be required. It would probably be worth speaking to the planning people at the local Municipality offices before committing yourself to any project.
I hope this helps.