Making sure your problem is a problem
First I would just load everything in the same scene all at once and see if that's actually a problem. 500 triangles is tiny for the interior of a building in the context of a modern computer, are you using high-res textures or targeting super low end devices? You might realize everything can be loaded simultaneously and synchronously and avoid these problems altogether.
Ok so there's a (memory) problem
Especially if you're targeting low end mobile devices, memory constraints may pose a problem. In that case you'll only want to load the interior of buildings when you need them.
I would try to intelligently load scenes in place asynchronously and additively first to avoid loading bars and maintain a sense of continuous space. But it depends on what type of game you're making if this is worth the technical complexity of a seamless experience. You could use Scenes to do this, or Asset References (using Asset Bundles).
Loading Bar 🏠 ▓▓▓░░░░░░ 🏡
If immersion and seamlessness aren't particularly important, then progress bars and synchronous loading are the simplest option. Maybe you actually want interiors to be bigger than they appear from the outside, in which case you'll have to do the character teleporting strategy anyways.