It seems that if you want a 15-minute city, the way to do it is to build the entire town in one building and minimize travel properly.
The Boeing Everett facility is 8 stories tall and just under 100 acres, it's a single building that's actually about as large as Disneyland for comparison, though it's hollow inside so it should be easier to make one that has floors and internal structural supports, and that factory was built using 60s technology. I suspect we could actually make it twice as high and also add several layers of basement but let's assume it's still only possible to go up 8 stories.
Making a similar structure but with floors would give us 34,394,976 square feet to work with. Let's presume one-quarter of that space is taken up by elevators, hallways, moving sidewalks, ducts, and some rooms that are two floors high for dramatic effect. 25,796,232 square feet left. Assume two-thirds of that is living quarters. We have 17,197,488 square feet of homes. Minimum apartment sizes for one person tend to be about 400 square feet (Japan goes smaller but that's Japan). A thousand square feet is generally considered extremely generous per person so let's go with that. About seventeen thousand people live in our mega-building. We can probably more reasonably round up to 20,000, with wealthier inhabitants taking the windowed wall apartments, the corners being the highest-prestige, and smaller low-budget apartments being deeper inside the complex where there are no windows or balconies.
Because it's all one building, it can be made more efficient, f'rex only soundproofing is really needed in between rooms and thermal insulation only needs to be on the outside walls, so we could go for broke with outer walls having five-foot thick insulation reducing heating to nil (we do have the body heat of 20,000 people) and making certain the summer heat won't be able to infiltrate and raise cooling bills in summer. Similarly, individual colossal heavily insulated air conditioners and heating systems for both and air and water will be more efficient than thousands of tiny ones.
The remaining quarter of the space gets devoted to commercial support. Shopping centers, research labs, theatres, restaurants, gyms, spas, bars, offices, etc. This is actually really excessive compared to the acreage most cities devote to this, and none of these structures would require parking lots, but we could have some light industry and manufacturing in there. Heavy industry is probably going to generate too much noise/heat/pollution. The roof of the building, being the size of Disneyland already, gets converted into an open-air amusement park or botanical garden. In some areas like New Mexico, it might be better to cover it with solar panels. This provides quite a few jobs already
For a distance of thirty-minutes in all directions, surround the building with outdoor attractions. Hiking trails, lakes for fishing and boating, biking trails, gardens, playgrounds, stadiums, tennis courts, waterparks, maybe even a NASCAR track going all the way around so that people on the roof and outer balconies can watch.
Past that zone is farmland, with roads and rail connecting multiple megacomplexes together, and a set of power plants sufficient to take care of both the farmland and the megacomplex. Farmers and ruralites live on their own land, only urbanites live in the megastructure. Spots with inferior land for farming instead contain factories, manufacturing complexes, mines, and other heavy industry providing blue-collar jobs.
Bam, five-minute cities that make fifteen-minute cities look wasteful. Minimal land used, generous amounts of space for all inhabitants, access to nature and entertainment of all kinds, and the building concept can be refined into a self-contained colony for hitting up Mars in the future.
