Selection of a plot in the new development area

well... there are sensitive people who would reject that, including having their garden there

That would not personally bother us, unless it demonstrably had some impact on health.

pretty accurate. The prices are basically within the usual range. Cellars as utility cellars. Residential basements have nearly similar costs to above-ground living space. If it is someone from your area who said that, then they will roughly know the regional prices and won’t lead you into a trap, nor expect golden faucets.

The colleague offers project management & planning construction supervision for service phases 1-8.
I can hardly estimate through which in-house work/self-contracting and general planning/implementation on the house potential savings can be realized.

For interior work such as electrical, flooring, tiles, sanitary, plasterer and tinsmith, I have private contacts.
To what extent these trades can potentially lower the €/sqm, I cannot imagine.

What do you mean by interesting? Many often plan a granny flat at a minimum size of 30 sqm to get funding. For grandparents, you should plan a two-family house right away, because who wants to accommodate their parents in 30 sqm, possibly still in the basement? Are they bringing money in or paying rent? Are the 250 sqm still affordable then? Such plan B options should already be known when choosing the plot, because 150 + 80 + double garage + carport or carports for 2 families, the plot should then be suitable for this. Possibly the semi-detached house plot would be interesting then.

The idea for the grandparents was that as soon as one party requires care, they would be accommodated in a separate living unit.
Friends of ours are currently renting out their granny flat on Airbnb, for example. The tax declaration is still unknown, but the demand is high.

...these are all still ideas, how meaningful it ultimately is and what it costs or which monetary benefit comes out of it is not entirely clear to us yet.

That would not personally bother us, unless it demonstrably had any effects on health.

There is nothing to fear health-wise. According to the plan, it really seems to go right through the middle of the garden; surely a right of way for the utility company will be required, with all the consequences for buildability and use. I would inform myself thoroughly beforehand about what to expect.

The colleague offers construction supervision and planning services for service phases 1-8.
I find it difficult to estimate through which own services/own contracts and general planning/implementation in the house potential savings can be realized.
For interior work such as electrical, flooring, tiles, sanitary, plasterer, and tinsmith, I have private contacts.
To what extent these trades can potentially reduce €/sqm, I cannot imagine.

Dear readers, please do not imitate! - even project and construction supervisors cannot perform miracles. The big issue with own services and craftsmen hired on your own is their timing in the construction process. A tiler who unfortunately only has time while the screed still has to dry is useless. Without supervision in service phases 6 and 7, there is also the added risk that as a layperson you will make expensive tendering mistakes.

The idea for the grandparents was that as soon as one party needs care, they would then be accommodated in a separate residential unit.

That is the worst possible time. Grandma caring for Grandpa in the basement, what nonsense.

Friends of ours are, for example, currently renting out their granny flat via Airbnb. [...] the demand is high.

So high that more and more municipalities are targeting this issue. It is definitely within the competence of a municipal council to prohibit such use by statute. Also, "retrospectively" in the sense that no restriction needs to have been mentioned yet in the development plan.

high.

...these are all still ideas, how sensible it is in the end and what it costs

But you can calculate the ideas yourself. Such thoughts are not about a bathtub or not with an additional expense of €1500, but simply about a number: €150,000, which you either have or don’t. The fact that the basement or an extension for a granny flat is possible should be manageable for yourself on graph paper with a little calculation without a calculator.

The idea for the grandparents was that as soon as one party needs care, they would then be accommodated in a separate living unit.

That also includes caring for the person? There is care allowance for that, but then a salary is also lower. If you think "in 20-30 years," you’ll have the children's rooms anyway.
….

Friends of ours are currently renting out their granny flat on Airbnb, for example. The tax return is still unknown, but the demand is high.

Yes, if that is allowed (development plan!) and one of you likes to always clean the little room afterwards, and often enough that it pays off... if you can provide a parking space and also open the garden to the tenant?! Many simply have no desire to go into debt at all just to be service providers for others…

Get clear on what you want – then choose the property.

I find it hard to estimate through which in-house services/self-assignments and general planning/implementation on the house, potential savings can be realized.

.. a value that doesn’t make an unspeakably big difference.

For interior work like electrical, flooring, tiles, sanitary, plasterer and tinsmith, I have private contacts.

But you still have to pay them when it’s contract work that should lead to a warranty. If you also want to receive subsidies, an adequate positioning of many certified works is necessary.

To what extent these trades can potentially lower €/sqm, I cannot imagine.

Well, from - to ... it’s already said with 3100-3500. If you take the here estimated €3000/sqm living space to compensate something like own contributions, e.g. painting work etc., you are also well off for calculating. But don’t be surprised then that the house construction takes longer and there is a double burden. Because such items should also be considered in a calculation.

Edit: look at the development plan of the residential area: accommodation trade is prohibited. Parking spaces for over 60 sqm apartment: 2 Smaller than 40 sqm 1 Means with granny flat at least 3 parking spaces

Dear readers, please do not try this yourselves! – even project and construction supervisors cannot perform miracles. The big catch with self-performed work and craftsmen hired on your own initiative is their timing in the construction process. A tiler who unfortunately only has time while the screed still has to dry is useless. Without a supervisor in performance phases 6 and 7, it also adds that as a layman you will make expensive tendering mistakes.

Thank you for the objections and the insights. What would be a typical example of a tendering mistake and the avoidable consequence?

That is the worst timing. Grandma takes care of Grandpa in the basement, what nonsense.

Our grandpa spent his last years in a converted hallway with the parents, as there was no room available. Hence the thought.

That also includes the care of the person? There is care allowance for that, but then there is less salary. If you think "in 20-30 years," then you will have the children's rooms spare anyway.

Both of my parents are single and over 70. But yes, probably a room on the ground floor and upper floor is enough here.

Get clear about what you want – then choose the plot accordingly.

Yes, the visibility distances here make you think twice again.

But then don’t be surprised that the house construction takes longer and there is a double burden. Because such positions should also be considered in a calculation.

Thanks here too for the food for thought, I will take that on board.

Edit: look into the development plan of the residential area: accommodation businesses are prohibited.
Parking spaces for apartments over 60 sqm: 2
Under 40 sqm: 1
Means at least 3 parking spaces with a granny flat

"The uses according to §4 paragraph 3 numbers 1-5 of the Land Utilization Ordinance (businesses of the accommodation industry, other non-disturbing commercial operations, administrative facilities, horticultural businesses, petrol stations) are not part of the development plan."

The assumption regarding the parking spaces was also spot on. 3 parking spaces on ~500 sqm seem a bit much at first glance.

Our grandfather spent his last years in a converted hallway at his parents' house, as no other room was available. Hence the idea.

Just because this can work in an emergency doesn't mean it should be planned that way. If you plan with the grandparents, they belong in an apartment that is barrier-free accessible and ideally also barrier-free inside. Logically, this is almost always on the ground floor.