So pretty much all of you have probably seen the channel, TheBackyardScientist. Hes a really skilled YouTuber that does all of these crazy science experiments including Defibrillating steak, playing with molten metal, and other crazy stuff. The Defibrillation and capacitors really got me thinking, 'Is it perfectly legal to buy a bunch of capacitors and make my own high voltage toy as long as i know what im doing? I typed "Is it illegal to own a TELSA", and the results came back as trying to convince me to buy a TESLA. I want to know if it is illegal to make my own Telsa Coil and use it, as long as it doesnt do any of the following: Disturb the neighbors, Injure anyone else, Not big and bulky, or turn into a bomb and blow everything in the surrounding area up.
The chief legal problem might be (depending on how you build and operate the thing) the amount of Electro-Magnetic Interference (EMI) that you're causing.
You are not exempt from FCC regulations, but
§ 15.23 Home-built devices. (a) Equipment authorization is not required for devices that are not marketed, are not constructed from a kit, and are built in quantities of five or less for personal use. (b) It is recognized that the individual builder of home-built equipment may not possess the means to perform the measurements for determining compliance with the regulations. In this case, the builder is expected to employ good engineering practices to meet the specified technical standards to the greatest extent practicable. The provisions of §15.5 apply to this equipment.
Since the question assumes "the builder knows what he's doing", we may assume the FCC demand "the builder is expected to employ good engineering practices" is met. But that also would assume that the builder knew about EMI in the first place.
If you live in an apartment or condominium, the lease or condo association agreement may have rules about what you are allowed to plug into electric power outlets. Such contracts might require that anything you plug in be listed by a testing laboratory such as UL or ETL. So consider this if the device you make plugs in.
It is almost legal, which means there are no laws or statutes prohibiting possession of capacitors or transformers. Whatever you make, even a 12V device, you may be liable for damage you cause, even unintended damage. Theoretically you could be held liable for patent infringement for building your own Tesla, but if they sue you, you may be able to assert that you had permission (hire an attorney before playing with fire). When you start doing things in the workplace, or selling products, then you have safety regulations to deal with, and certain kinds of monkeying with house wiring requires a permit and an inspection, but that doesn't seems to be what you have in mind.