The municipality determines the rules. For example, one municipality may require that the minimum construction is 110 square meters and the maximum is 30% of the lot area. You can build up to 2 floors. On lots of 4,000 square meters, you can build the main house and a guest house, as long as it is less than 600 square feet and does not have a kitchen. They also cannot be divided, so if you ever sell your lot, you have to sell it with the 2 houses.