Approximately 25 m² extension with a walkable roof

Hello everyone,
we are considering moving into my parents'/grandparents' house, and as is often the case: there isn’t enough space.
Therefore, we want to build an extension of about 25m² (not basemented) and at the same time use the roof as a roof terrace for the apartment on the first floor.
My calculation:
about €4000 additional costs (architect, surveying, ...)
about €40,000 extension
Cost of roof?
Cost of spiral staircase to the terrace?

Are these values realistic and what about the other items?

PS: We are still at the very beginning of the planning and grateful for any tips.

Hmm, it could be feasible but doesn't have to be, it depends on too many parameters, generally you can build new from €900/m², but it can also easily cost several thousand €/m².

What about:
-Building permit?
-Wall structure?
-Windows (wood, plastic, double or multiple glazing....)
- Heating?
-Sanitary facilities or kitchen? For both, you can quickly spend €40,000
and
and
and

Hello,

That’s why we want to add about 25m² (not basemented) and use the roof at the same time as a roof terrace for the apartment on the first floor.
My calculation: about €4000 additional costs (architect, surveying, ...), about €40,000 extension, roof costs?, costs of spiral staircase to the terrace? Are these values realistic and what about the other items?

Before you start thinking about possible costs, I would first go to the building planning office responsible for you and explore the chances of a possible extension. There you will also find out within which limits a roof terrace is allowed.

Then you need – besides an architect for the building application – also a structural engineer who calculates the load capacity of the flat roof; includes the existing structure in the planning. Only with knowledge of all parameters can you ask a general contractor or individual tradespeople for an offer.

No one here can give you a reasonable forecast without knowledge of the property conditions => whether access is possible without effort, etc.; what I can tell you is that – the smaller a new building is planned – the disproportionately higher the costs have to be estimated. You can safely forget the number/sqm/extension suggested by my predecessor.

Kind regards

You can safely forget the figure/sqm/extension mentioned by my previous poster.

I have clearly pointed out that you cannot say that, and from 900€/m² you can calculate (if everything fits perfectly, you provide a lot of your own work, build the cheapest of the cheap, and preferably live in a federal state with low wage levels) nothing more! But as I said, it’s like looking into a crystal ball, and the project can easily become 4-6T€/m² expensive.