Question about expanding a single-family house into a two-family house

Greetings!

I'll jump right in with a question.

My wife and I (29 & 31, currently 1 child, second child on the way) are planning the expansion of her parents' house. On the west side of the house there is currently a passageway (with a pellet tank) as well as a garage adjoining it. Everything is covered, of course. On top of the garage/passageway there is currently a solar system for hot water.

The plan is to add a double garage so that additional living space can be created above the garages and the passageway. Her parents will then live on the ground floor, while the entire upper floor would be available to us. The upper floor has 3 rooms including a bathroom & toilet and measures approximately 8 m x 8 m. On the west side there is a window in the hallway which is to be opened up, thus creating a passage to the "new" area. The new area would currently have external dimensions of 13 x 6 (alternatively 13 x 7). This will include another bedroom as well as a living/kitchen combination (and a small storage room). The entrance on the ground floor is currently also on the west side. Our entrance should then be installed on the north side (probably via a staircase).

Thus we would come to about ~120 to 130 sqm of living space on the upper floor.

We want to build the double garage solidly.

Now the question arises whether we should build the upper floor solidly or as a wooden structure. What would be the advantages? Are there, for example, any price differences? We have not yet obtained any offers or similar. Also to be clarified is a possible land expansion (which is possible to the north as well as to the west).

If I should describe anything in more detail, I am of course happy to answer.

Plans say more than a thousand words ...o_O

About the plans.

Yes, of course I could add those.
But I would have to scan them first. (and I am currently on vacation)
They are of course already much further. They are currently obtaining various offers (2 of them solid construction, 2 with solid wood)

Something else
This concerns the budget

What we need is a foundation slab as well as a double garage. On top of the double garage and an existing garage, we will create about 75 sqm of living space.
The integration with the "old building" seems according to all construction companies to be very easy. (There will be a transition from old to new building, whereby only 1 window will be replaced because a passage will basically be created there)

Although the old building may already be 40 years old, everything that is important there is practically as good as new. (heating, windows, toilets, floors on the ground floor, kitchen)
The first floor of the old building will therefore be part of the new building. Besides the bathroom, nothing needs to be renovated (the toilet was renovated the year before last)
Possibly later the floors upstairs will also need to be renovated.

The first floor in the "old building" already has 3 rooms, 1 bathroom, and a toilet
The new building with its 75 sqm includes kitchen, living room, storage room, and 1 additional room
Thus, we would have about 130 sqm of living space on the first floor (about 60 sqm on the ground floor ... which will be used by the parents-in-law)

So briefly again the points of what we want to build
- 75 sqm new building (tending towards the solid wood variant)
- double garage
- wider driveway (new and wider paving, etc.)
- renewal of the solar system (old one removed, it’s more than 20 years old - only needed for hot water -> so we need about 10 sqm)

As a total budget, we have about ~ 250 TEUR
Whereby we plan to spend "only" about 200 TEUR on the above points.

The rest that remains we will then need for fees, notary, incidental costs, the new kitchen, the renovation of the bathroom as well as optionally the renovation of the floors and doors in the old area.
If there is still money left, one could consider immediately reroofing the roof in the "old area"

Is that enough? Could that work?
Are we overlooking something?

Simply guessing like that is absolutely not possible to judge.

So briefly again the points of what we want to build - 75 sqm new building (tendency towards the solid wood variant) - double garage

Double garage ~ 30K Euro

- wider driveway (new as well as wider paving, etc.)

Estimate about 7 meters x 6 meters

42 sqm x 100 Euro / sqm = 4,200 Euro

- renewal of the solar system (old one removed, it’s already older than 20 years – only needed for hot water -> so we need about 10 sqm)

cancel or put at the very end

As total budget we have about ~ 250K EUR

In total, you create 130 sqm of new living space. I would estimate 1,200 Euro per sqm of living space here. You don’t need certain things. Roof stays on, heating is inside. Doors and stairs too. Some windows new, electrical and sanitary will come.

156K Euro

Planning costs architect + structural engineer 20K Euro

Secondary construction costs hardly apply, connections are in place. surely no crane needed and groundworks are also manageable.

Roughly, I also come to 210,000 Euro

As said, just a rough plan. Maybe some can say more. Such an extension often also involves uncertainties. What about the building permit?

With kitchen, small stuff, notary, fees you will quickly reach the 250K. [/QUOTE]

Have you already asked the building authority whether that is possible and permitted?