You usually need a building permit before starting construction. The exception is the construction of a building of less than 30 square metres for which you do not need a building permit. For example, you can build a sauna cottage without applying for a permit. Although a building permit is no longer necessary, it is important to remember that the building must still comply with all valid provisions and regulations.
Apply for a building permit at Lupapiste (permit point, in Finnish) or from the municipal building supervision authority.