alt Hacker News> It’s more than just extra fixtures and interior walls but bathrooms, kitchens, plumbing, and HVAC - some of the most expensive parts of a build.
That isn't necessarily saving anything. A 4000 square foot unit could very plausibly have four baths and HVAC zones, especially if it's a luxury unit.
Meanwhile not doing that is exactly the sort of thing the existing rules nefariously prohibit. Suppose you want to turn that 4000 square feet into eight studio units with a dorm-style shared kitchen and bathrooms. Now you've significantly lowered your construction costs while increasing the total you can sell for because you're offering eight units, so that becomes the most profitable thing as long as there are enough people who want lower rents more than space, except that hardly anywhere allows you to do it that way.
A developer in our example would still profit by offering a luxury unit with 3 bathrooms instead of 4. But you’re right that there will often be multiple baths and HVAC systems in a luxury unit - although electrical, plumbing, and fire suppression systems still scale up with the number of units.
And kitchens remain a problem. Your dorm-style kitchen layout is creative, but the problem with this idea is that there is very little demand for dorm-style housing. Developers are able to charge higher prices for more conventional layouts - where the demand is - which is what they will build even in the absence of regulation.
Markets can handle some things well but the inability to provide merit goods (like housing) is one of their shortcomings. Unfortunately we need policies with more scope and ambition than YIMBYism if we want to tackle society’s larger problems.