Development plan - Symbol "Triangle with inscribed D"

No, not always. For example, an "o" without a triangle can also be included there.

Where "o" stands for "open construction." That is something different again.

What should I do with a duplex?
One is enough for me

"D" means more precisely that you have to build one half of a duplex house there, i.e. attached to the neighbor on one side of the property. It does not mean that you would have to put a whole duplex on your property :-)

There is no neighbor on a neighboring property...

There is no neighbor on a neighboring plot...

In this sense, it means that you have to build up to your shared boundary. To the left or to the right neighbor, you choose first ;-)

Where "o" stands for "open design". That's something different again

Namely?
In my opinion, an umbrella term.

That is original: All around there are only detached houses, alone and lonely.

Namely?
Imho a generic term.

And in that respect it makes sense if it itself is written differently than its more specific terms.

@TE: that means "o" includes E, D, and even H (house groups, "row houses").

That's original: all around there are only detached houses, alone and lonely.

If reality mostly consists of exceptions, the development plan should actually be revised.