Commercial buildings can meet the five requirements in a different balance than rural land.
Seclusion is satisfied by walls, controlled use of windows, and access control through doors. Many commercial buildings have some level of security designed into them. Whether or not the building is publicly accessible, access is generally limited to specific entry points and a lobby controls access beyond that. Fire exits are locked from the outside. There are no other easy ways into a building. For rural land, a hike through the woods can probably get you in. It isn't practical to put a fence all the way around a large campground or resort. Therefore, a commercial building in a city is better secluded than rural land.
Sunshine and Space are the most limiting factors with a commercial building. The building has a fixed size, and it has a roof which blocks all or nearly all of the sunlight. However, both can be increased by a fenced in area outside or by fencing in the roof (if it's flat). Consider a green roof for a naturalistic roof development.
Accessibility is a strong feature of a commercial building. The location can be inside or on the outskirts of a city. Not only is it a short drive for everyone in the city, but there is likely cab service to the local airport too. Everyone in the county (or beyond) visits the city to do shopping, go to appointments, visit friends, and whatever else. They could stop by the venue for a couple of hours if they have time to kill. Nudism would be casual experience instead of a planned vacation or second residence. I'll talk more about this in part 5.
There is no doubt that buildings are more Comfortable than the outdoors. We go through a lot of effort to build them, maintain them, keep them the perfect temperature, control humidity, keep the air fresh, keep the weather and bugs out, provide lighting, maintain security and privacy, provide clean running water, retain food for long periods of time, provide ways to prepare that food, and have a safe and comfortable place to get a good night's sleep. Traditional nudist venues have some level of difficulty providing all these things and are often at the mercy of the outdoors.
Recently, I've been imagining what I'd do to convert various commercial buildings into nudist venues. Depending on the building, it might be easier than you think.
A friend of mine used to live in an old schoolhouse. It was a big square three story building within the city that was developed into apartments. Each classroom became an apartment. They kept the small gym as a recreation area, and added a pool within it. Access to the building was controlled through a single door. It's not hard to imagine this as a nudist venue with the pool and rec. area, and with roomy rentable hotel-style rooms.
Buying an old hotel can provide the same setup but with less work. Rooms are available to rent and there is often a nice pool and recreation area. Access control is already in place.
There are a lot of closed factories, stores, churches, and office buildings around. Many have ceilings high enough to not disturb a volleyball game, and many have offices that can be rental suites. Some will be more work than others to convert. Every building is different. Each has it's own layout, it's own maintenance costs, and it's own price tag.
In the next parts I'll discuss layouts and locations for an urban nudist venue. After that, I'll explain why this alternative approach to nudism might just catch on.