Roof expansion reserve - requirements and costs

Hello everyone,

the following situation: Our development plan allows for 2 full floors. This means we could design the roof so that we can later convert it if desired. But what does this mean concretely? Does anyone have experience or reference values regarding what creating a conversion reserve entails financially and technically? What must already be established now as a basis on top of the following factors:

- Staircase to the attic (maybe even installable afterwards if you only create the ceiling opening now?)
- Windows on the roof
- Routing of cables to the roof

Thank you very much in advance for your experiences and assessments.

Stairs to the attic. Positioning may be difficult due to the sloping roof. Would install immediately

Rafter insulation instead of ceiling

Empty conduits and 2 light bulbs are enough Storage, play area, etc. Friends have moved the drum set upstairs

Airtight door

Possibly lay screed already

Window in the gable