This construction project seems to me like a harebrained idea of the highest order. Either the "basement" with its generously exposed front can only be built with significant terrain engineering work (in which case the terrain intervention itself would already be a construction measure requiring approval) or it can be built with the existing terrain (in which case one would probably have to calculate carefully whether it does not constitute a full storey). The way of thinking that if you are only allowed to build a single storey, you simply choose a nice floor height for it, call it the "only storey," and everything below it should then be considered invisible for calculation purposes, is wishful thinking.
A weekend house area is a weekend house area remains a weekend house area, even without red cabbage. The excerpt shown in post #57 confirms that this will not be altered even by a hundred first-rate lawyers. With the also legally enforceable attic, Katja is probably right, but also with her assumptions regarding the municipality's public service obligations. The permanent residence ban implies that the municipality is exempt from providing wastewater and waste disposal appropriate for permanent residence; and moreover, I even consider an exemption of the municipality and residents from corresponding traffic safety duties (street lighting, winter service) to be derivable from this. One could even go further: if the tolerance of the illegal permanent residence (which in my view already constitutes perversion of justice) is extended by organizing wastewater and waste disposal appropriate for permanent residence thus further facilitating legal violations, I see independently punishable conduct as completed.
Perhaps the OP is relocating the story safely synonymously to "might just be made up" or "you'll never find the place" to "Bielefeld."
Building planning law must take landscape and other environmental protection into account and weigh them against housing needs. Where a blending between inner and outer areas appears appropriate, allotment gardens and holiday home developments may be a suitable instrument. Their conversion into permanent residential areas can be denied to a municipality as a result of the weighing process, which the district as the responsible nature conservation authority must then also convey to it. Exceptions to regulations may only be granted at equitable discretion. Both that a building authority does not intervene against illegal land uses would at least violate the official oath, and a tacit secret amendment of the development plan would no longer be just unfair but rather outright illegal. We would be talking here—hence the hope for pure fiction to entertain a popcorn thread audience—about violations that go beyond the scope of fines or probation and must result in the removal of the (failure to act) offenders from civil service.
In this sense, a happy "and if they haven't died, then they are still living today" (in Schilda, Lummerland, or Bielefeld), may this have been an entertaining lecture on building planning law for guest auditors!
Since I suspect mostly laypeople in the audience and am not a lawyer myself, I have spared the mention of the relevant legal norms and their references. After all, we are not here to work. For those interested in the tension between spatial planning and municipal council engagement, I recommend seminars offered by their respective state centers for political education (the by far more interesting part of spatial planning is beneath the federal level).
I thank the OP for the excellently authentic illustrations including the dotted line of winkingly unknown meaning (perhaps Gaston peed there). We have had many purely textually narrated fantasies about outer area residence here; now one has at last been illustrated. I'll drink to that a Dujardin!