Is only part of the building permit enforceable? How long is the building permit valid?

Hello experts and all other forum members! I am new here and concerning building law and would appreciate help.

We will acquire a plot at the edge of town. It is no longer included in the development plan and §34 Baugesetzbuch applies (therefore I assume it is within the inner area). According to the realtor, there is a building inquiry and a building window. It is a small village (in the middle of nowhere) in Rhineland-Palatinate (just for background - perhaps helpful regarding how authorities and the municipality decide). The neighboring house is 2.5 stories high and has a footprint of about 220 sqm.

Our plan is to build a large single-family house with a granny flat, which will one day be available for elderly parents (rental is not intended). However, since we currently cannot or do not want to afford the desired single-family house (bank dependency and such), our plan is to first only build the granny flat. (This should be at the edge of the house anyway). The plan is therefore to move into this 50 to a maximum of 80 sqm granny flat and eventually, when we can (financially) and want (I would like to do a lot myself - would be a dream), build the rest of the house. This could be in 1-2 years or in the worst case never. The granny flat is to be 1 or 2 stories and have a basement.

Two questions I could not clarify through research: 1) Is it possible to apply with the architect for the approval of the entire desired house but then only build the granny flat (this is a separate house, just not the large desired house)? To build the remaining part of the house sometime later or never? I could only find information about when construction must begin and when it must be finished (not specific to this municipality). It is not a new development area.

2) Assuming 1) is possible, we plan to have the basement already built for the entire house. So about 40 to 50 sqm under the granny flat and another 100 sqm for the planned house. This 100 sqm basement part could therefore stand alone for years or forever without a house on top. Would something like this be approved and does the ceiling have to be specially sealed?

Any advice helps and we are grateful for it!

For the validity of the building permit see Landesbauordnung RLP § 74: (1) The building permit and the partial building permit expire if construction work on the project has not begun within four years after its delivery or if the construction has been interrupted for four years. The execution of a project is only considered to have begun or not been interrupted if significant construction work has been carried out within the period. (2) The period according to paragraph 1 can be extended by written application each time for up to four years. It can also be extended retroactively if the application was received by the building supervisory authority before the deadline. The extension may be associated with new ancillary conditions. The decision on the extension shall be served to the builder; § 70 paragraphs 4 and 5 apply accordingly. Now you can decide for yourself what to do. Not building costs a lot of money for the architect. Why not plan twice – and only build the big house when you actually need it? I don’t quite understand the background of the question.

Is it possible to completely separate the [Einliegerwohnung] from the house?

Is it possible to completely separate the granny flat from the house?

If the granny flat is the ground floor and the rest is relocated to the upper floor or attached as a duplex, yes.
Nevertheless, it becomes difficult if you get approval for two stories (because you have to), but only build one story... then there is a problem.
There is also the problem that the technology is more linked to the main apartment.
I think all of that would be too many compromises to make in the planning.

How about having the entire shell construction quoted as a "shell house" and then only finishing the granny flat?

According to the realtor, there is a preliminary building inquiry and a building window. It is a small village (in the middle of nowhere) in Rhineland-Palatinate (just for background

In the land of turnips and vines, villages are actually only in the middle of nowhere. By the way, there is a rumor among realtors that everything they don’t sign is a lie

However, since we currently cannot or do not want to afford the desired single-family house (bank dependency and so on), our plan is to first build just the granny flat.

Especially in the variant of placing a basement for the later-or-never core house as a "Schürmann building" next to this annex/semi-detached house, the project sounds suspiciously close to a crackpot idea in my opinion. You would have to pay the architect for the entire structure, which from the perspective of the still missing use you would “pre-finance” just like the unused plot share.
If your calculation doesn’t work out and the plot with the house falls to the bank, it won’t become more attractive because of the ghost cellar. But to the core question: I see the building permit reaching its expiration date before the continuation of construction.