Have you ever considered getting one or two cost estimates for the civil engineering work? The terrain is not that simple; semi-trailers/excavators/trailers also need to get there, meaning you need a construction road. You can't really judge that just like that. We just don't want you to miscalculate with the 100k€. It might go well, but it might not. Maybe you need retaining walls, maybe a lot of earth has to be removed because it's unsuitable as a base, maybe a lot needs to be filled and compacted, maybe there is bedrock in the way. And the utilities are also hard to estimate from the photo. There's a huge cost difference between 5m, 30m, and 300m of sewage/water/electricity lines. Maybe you have to divert rainwater separately, maybe temporarily store it in a cistern... it's a bit of a gamble. You should probably inform yourself first. You don’t want these ancillary construction costs to throw a wrench in the works.
Many useful things have already been mentioned to you. In principle, you can save on everything related to electronics. But also regarding aesthetics. You pay a lot more for anthracite windows, for example, plastic windows are cheaper than aluminum/wood. There is actually enough reading material on the internet; this isn’t exactly new science. But if you need help for your specific situation, you’ll need the most important information for that as well.
You can save on:
- roller blinds instead of venetian blinds (possibly not necessary in certain rooms at all)
- no large window fronts or sliding/lift-and-slide doors (possibly additional steel lintels necessary)
- decentralized instead of central ventilation system
- with us, 24cm wall + 16cm insulation is cheaper than 36cm wall
- no KfW40 house or higher
- no smart home, manual room thermostats, roller blinds operated by belt
- heat pump from Daikin instead of Wolf, for example
- smaller hot water tank as only 2 people – Hamburg
- fewer windows/roller blinds
- Q2 plaster instead of Q3, do the rest yourself with plaster/sanding, possibly just paint (without fleece)
- smaller tiles under 80x80 or little tiled area
- no extra shower bathroom or plan only one shower
- cheap interior doors instead of solid wood (DIY installation)
- laminate/linoleum
- concrete roof tiles instead of clay
- no colorful additional fittings like on windows/gutters etc. possible
- exterior paint white at no extra cost
- beech staircase instead of oak/concrete/steel
- forgo TV sockets and watch TV via DSL (lay LAN cables yourself)
- pay attention to the number of sockets/switches, no video intercom, no surveillance cameras, no fingerprint locks
- forgo a fireplace
- aluminum window sills outside instead of stone
- normal base color instead of base plaster
- drains as close together and as few as necessary
- water pipes for bath/toilet/kitchen near the utility room
- low knee wall is cheaper than building it up
- few or no skylights
And so on and so forth. You probably already know most of it.