House planning - thought experiment possible third child

Underfloor heating would be possible if you use a dry screed. However, the floor construction then reduces the ceiling height again, which might be critical in the attic where every centimeter counts. Therefore, despite the air-to-water heat pump, we decided on regular radiators.

: What do you roughly have to budget for the preparations for an expandable attic? Additional staircase, additional supply lines, stable floor... that surely adds up.. can you give a rough estimate?

Manu has given you the right information twice; thanks Manu! However, I estimate EUR 15,000 (for your desired size) including the continuation of the original stair flight. Additionally: You can assume that for a preparation for expansion including continuation of the stair flight, insulation of the sloping roof and stairwell, living room door, window of escape route dimension, continuation/capping above the top edge of the ceiling of all lines as well as full-surface boarding for a house of the desired size, about €7,500-8,000 will be incurred. If you build out the attic from the start and additionally want to install underfloor heating in this area, you can solve this with dry screed above a wooden beam ceiling over the ground floor or a concrete ceiling above the ground floor. Rhineland regards