Dig a pit for the after-bath pool and lay the foundation in concrete. Cover the floor in long, flat planks of aspen wood and tile the pool in ceramic. Mark where the walls will divide the pre-bath, sauna and post-bath rooms. Set the sauna room adjacent to the pre-bath.
Mortar the furnace, chimney and coal bowl along the dividing line of the pre-bath and sauna, with the coal bowl on the side of the sauna and the furnace and chimney in the pre-bath where it can heat the room. Leave an opening between the fire pit and the space underneath the coal bowl so that the fire can heat the coals in the sauna. Bolt the iron door onto the furnace opening.
Build walls in and around the banya with logs of aspen wood. Make sure the logs are packed together tightly to keep the chambers well-insulated. Board the ceiling with planks to encapsulate the structure.
Build benches into the wall of the sauna at different levels of height so that you can move among varied temperatures and comfort levels. Furnish the pre-bath chamber with standalone benches and a table, whatever furniture you think will maximize your comfort. Fill your pool; it should stay cool located low to the ground and in a room separate from the heating source.