Cost Assessment for Extension in RLP

Hello everyone,

I would need your assessment regarding the costs of an extension of a house with a gable roof. Although we already have our own experiences with new construction, core renovation, and renovation of a house from the 90s, we have not yet done an extension and thus have no experience.

Basic data:

The base is a 1.5-story house with a gable roof from 1981
Living area 195m2, currently we live on 320m2, so we are used to a lot of space. Unfortunately, we have to move 450km away. Floor area 14.5m x 11m
24cm Poroton with cavity and facing brick
Heating is new

Plan:
Extension to 2 full stories with corresponding enlargement of the living area.
Flat roof with photovoltaic system

In the remaining ground floor practically a core renovation:
New electricity
Plastering of bricks
New windows on the ground floor
New parquet floors
New bathrooms
New kitchen
New roller shutters

The layout upstairs is okay and the walls could basically remain if it makes sense or should everything be redone right away?

For the renovation of the ground floor I estimate about 1500 Euro/m2 = 225,000 Euro
Does anyone have experience how high the costs for an extension are approximately? Finished standard without electricity, so that I would only additionally need painter, parquet layers with material and sanitary for the bathroom.
My estimate would be that I am between 2,000-2,500 Euro/m2. All-in at 3,000 Euro with good standard.

In total about 700,000 Euro for renovation and extension.
Is that okay or am I way off?

We are therefore talking about replacing the attic with an upper floor and about a potential gain in living space in the order of 60 sqm. The procedure would technically, in theory, be to strip the roof, dismantle the roof structure, and remove the facing shell up to the current eaves height, in order to basically reach the ground floor ceiling as the "floor slab" of the new upper floor; the interior walls of the current attic would also have to be dismantled accordingly. Statics would have to consider that the new interior walls of the now upper floor would have to be lightweight walls and that the roof would have to span freely as a truss structure over the exterior walls, supporting itself. I do not contradict the assumption that this would be an economically luxury operation ;-)

If the attic is already converted now, you won't have more space later. Only the ceiling height is the same everywhere. Dormers would be cheaper. I also do not disagree with the claim that it is an economic luxury. A full renovation costs as much as a new build.

with the increase of 60sqm to the immense costs, the first option for me would be to sell the house and build or buy a new one in the appropriate size

First offer for turnkey is 2700€/m2.

First turnkey offer is at €2700/m2.

Does this only refer to the "new" living space from the extension or is this to be seen as an average for the renovation including the extension, etc.?