How much does our planned house extension cost?

Hello, who can answer the following questions for me: What does the following house extension cost? (Approximately estimated)

-basement slab and double garage (ground floor) already existing (assuming the basement slab is suitable for this) -on top of this, 2 rooms, kitchen, and bathroom should be built in a size of 8x5m (i.e. 1st floor) -walkable steel carport with external stairs as a separate entrance -roof tapering upwards with wooden cladding (without attic) with glass front in the living room

What is needed: -1st floor slab as floor (or ceiling of the garage) -masonry work -roof -windows -carport -building permit? -heating connection to existing gas boiler -connection for kitchen and bathroom

Does anyone have experience with what something like this can cost, may cost, or will cost? What else must be considered? Unfortunately, I am not very experienced. Thank you very much

What it costs is easy to answer: money. How much is much more difficult, as I don't quite understand exactly what you actually want to build now. You want to build 2 rooms on the existing garage? As well as a steel carport with a steel staircase to reach these two rooms? And what else?

For this construction project requiring a building permit, you should seek advice from an architect, without whom it will not be possible. He will first clarify whether your construction project is permissible under building and planning law at all or under which conditions. If necessary, a building inquiry must be submitted. Then he should check in a building survey whether the existing garage shell even allows for an extension - keyword dimensioning of the foundations / permissible ground pressure / frost protection. Once these crucial basic questions are clarified, he can prepare a preliminary draft for the extension including a cost estimate. Before that, any statements about possible construction costs are purely speculative.

I have to sort out the whole mess first:

-foundation slab and double garage (ground floor) already exist (it is assumed that the foundation slab is suitable for this) -on top of that, two rooms, kitchen, and bathroom in 8x5m size are to be built (i.e. 1st floor) - [...] So what is needed: -foundation slab 1st floor as floor (or ceiling of the garage)

means: there is already a foundation slab for a double garage, whose walls are already standing, but it does not have a roof yet (or currently a pitched roof), now a roof is to be built on it that at the same time is the floor for a small apartment above (?)

walkable steel carport with external stairway as separate entrance

I don’t quite classify the carport here: it is supposed to be a third parking space next to it (?) -"walkable" means a terrace on top for the small apartment, the outside stairs next to the carport are supposed to provide access to this small apartment (?)

roof tapering to the top with wood cladding (without attic) with glass front in the living room

so a gable roof with wood-clad open roof underside, and the living area of the small apartment with glass front – as a conservatory on the carport (?)

heating connection to existing gas boiler - connection kitchen and bathroom

that would mean the small apartment is also connected to the main house’s gas meter (?)

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If the garage is a boundary garage, I would already see that as an obstacle to building up a living space.

I also have doubts whether the 2016 Energy Saving Ordinance supports your ideas about this structural design.

Do I guess right that here an apartment is to be created over the garage for a grown-up child?

"Building permit" unfortunately means more than just registering your building wishes properly. Unfortunately, it also means that regulations demand higher building standards than those you might settle for. A cute treehouse is unfortunately not very regulation-compatible. Building authorities only know houses without monkeys and horses :-(