It's not the architect or design that costs. Building 5 identical houses next to each other is simply much cheaper and faster due to the economy of scales of prefabricating and delivering identical elements and being able to use the same machinery and crew to put them all up att once. Spreading the out over 5 postcodes means you need more people, more construction equipment, more infrastructure work etc. etc.

Maintenance over time is also easier and cheaper if the buildings are standardized.