Is it feasible to build a two-family house with parents / parents-in-law?

For which advantages do you prefer a "prefabricated" house?
Almost all of their manufacturers do not make their own basements. Due to the responsibility for fitting at the OKKD interface, make sure to have the basement supplied by the house manufacturer anyway (sometimes the manufacturers or even the builders themselves get the idea to order the basement separately).

It was no problem for us. Basement company X, prefabricated house company Y. X did not know Y.
The saws are only started once the basement has been accepted by the site manager of the prefabricated house company.
(Breaking ground 08.11.2008…building the basement…..20.12.2008 the "wooden hut" was already standing, moving in 31.03.2009)

Financing should be advised by others. I just want to put a question out there, are you both absolutely sure that living together with the parents or parents-in-law is okay for everyone involved?

mfg

I would advise against a prefabricated house, but that's a matter of taste. Wooden walls wrapped in plastic foil, and if water ever gets in somewhere, the whole place will rot away without you noticing.

I would recommend planning the parents' apartment as an independent unit, meaning with separate meters, its own building services (except maybe heating), and a thick partition wall, as is common in a two-family house.
I assume the parents won't live to be 150 years old and that you might eventually have to find tenants for it if you can't or don't want to afford the whole house. Often, it also doesn't work out with the children and you end up as just two people...
If you then have strangers renting in the house and there's a thin wooden wall between the apartments, you won't enjoy living there anymore. The same applies to sharing the utility room, etc.
Even worse: you do a real division of the property, the apartment is divided among the heirs after the parents' death (if there are several), they won't agree, the apartment must be sold, and then you have total strangers living in a single-family house planned for one family with timber frame walls. I'm sure all of this has happened before.

When dividing, I would personally consider granting a lifelong, non-inheritable right of residence and usufruct. I would also have the parents live in the basement with a nice little garden. Upstairs, on one floor, you with a garden facing away. That should actually be possible. The residential units should indeed be truly divided, with separate meters. Saves trouble later on. If the parents are no longer in the house and the children have grown up (if it even works out with children), you can nicely move into the cozy basement yourself and rent out the upstairs. For financing: keep in mind that salary increases might still come. Three years is a long time. Until it really starts, additional equity can be built up. However, construction costs will also rise.

I would personally consider granting a lifelong, non-inheritable right of residence and usufruct in the division.

But subordinated to the mortgage - thus the right would be lost in a forced sale, which would cost the parents the apartment -, otherwise you have a big problem with the financing.

True, completely overlooked. But a stairlift would be cheaper than an extra floor as a storage room.
Can you elaborate on that?

A stairlift for a wheelchair doesn’t cost just a few euros and also requires space.