Expansion costs for a single-family house starting from the shell construction

Isn't the question easier to answer the other way around: 1. How much does a house according to your ideas cost approximately 2. And what value can be deducted for the current condition? Question 1 you can probably answer best yourself, what you want. Question 2 can certainly be estimated by your expert, since he has seen the object and has the appropriate knowledge as an expert for this.

I started almost exactly as you describe... however, there was already a roof and plaster on the outside - but we had to "go back" because we changed the window dimensions. 100m² living space, house footprint about 70m², so "small". Without exterior work, I came to 120k€. I can't break everything down separately by material and labor... Here is my list (almost all net):

Windows (PVC with aluminum exterior shell) including installation 16k€
Carpenter (intermediate ceiling on the upper floor, one roof window, counter battens, partial drywall) 8.2k€
Material (interior walls, drywall, various insulation) 5.9k€
Construction supervision 1.5k€ (very cheap due to good connections)
Masonry of interior walls 1.4k€
Core drilling for pipes inside the house and house connections 0.9k€
Electrical work 9k€
Various small items (tools, various small materials, mailbox, doorbell button, endless others...) 1.2k€ (gross)
Interior plaster 4.9k€
Exterior plaster 8k€ (however, I only had repairs amounting to 3.9k€ since it was already plastered)
Sanitary and heating 17k€
Civil engineering 10.5k€ (handover shafts, house connections, but also base renovation, demolition of an unapproved part of the slab)
Exterior facilities (paths to the house, parking space, splash guard and soil for the garden) 7.5k€
House connections (fees from utility providers - gas, electricity, water, sewage, telecom) 4.7k€
Doors 2.4k€ (gross)
Material (wallpaper, tiles, adhesive, paint...) 2.2k€
Drywall filling, sanding, walls sanded ready for painting, wallpapering, painting 4.3k€
Bathroom tiles 1.9k€
Floors (tiled only bathroom, toilet, utility room) 3.7k€ (gross)

The doors were actually from the hardware store, except for a glass door for the living room. The tiles from the tile center, but all for prices around 30€ per m² (and they really are not ugly!!)
Own work: drywall (mostly), floor insulation, partly laying floors, assistance with masonry of interior walls

Addition:

forgot: staircase 3.5k€

(Floors were also around €30/m²)

With us, it was, I believe, around 200,000 in the calculation from the shell construction. Although broken down to the last socket, it was not divided into material and labor time. Status 2017 Approximately 200 sqm Is without underfloor heating and ventilation system

I'm looking for a rough estimate of the costs for finishing a single-family house starting from the shell.

Calculate between €1,200 and €1,700 per sqm, depending on the desired level of equipment.

We all don't know what you want to build, so we can't estimate.
A bathroom can cost between €5,000 and €50,000. Should the plaster be Q1 or Q4? Windows wood, plastic, or aluminum with which U-values? Interior doors plastic, veneered, or solid wood? Heating oil with radiators or underfloor heating with geothermal energy? Italian marble or DIY store tiles in 20x20? KNX? Electrical work simple or elaborate?

How many equipment questions should we ask in order to give you a price?

Electrical work only standard, no smart home, and I plan 15,000 euros for the bathroom, as well as additionally for the kitchen. The heating should be underfloor heating with geothermal energy (trench collectors) and additionally a fireplace for a small wood stove.