Property acquisition tax on a house not yet built - yes/no?

It's too late now anyway, but if the matter had somehow been settled "between neighbors" with a handshake instead of formally, a lot of money could have been saved. On the other hand, you can also understand the seller. Ultimately, it's another proof that the real estate transfer tax is not really regulated fairly. Because the construction company is obviously not the developer, they just cleverly told the seller what advantages a joint construction would have.

they only briefly told the seller what advantages a joint construction would have.

...and it is not entirely far-fetched that the "first builder" was sweetened the deal (with discounts or the like) if he immediately brought in the customer for the second half.