Can you put a doorknob that uses a key indoors?

Sure. Absolutely you “can”.

The better question might be “Do you really want to?” In case of fire or other necessary emergency access requirements to the room, you could cost valuable time hunting down a key or trying to break down the door.

I wouldn’t recommend keyed access inside a residence unless this was for a separate apartment or other separate living quarters – where another access is also available.

You may want to consult with your local building code enforcement whether the local building code allows this. Though I’m not aware of any such restrictions.

Yes. This is done often.

Yep. I have one on my office door. It was originally intended to keep the kids out, but now that they are older and reasonably civilized they have computer privileges in there (contingent upon them remaining well mannered, of course)

Yes, as long as it is not a bedroom, or any other room used for sleeping.

Of course.

I’m assuming you’re talking about a deadbolt lock.

Why would you even want one which requires a key rather than the ones designed with the flip switch (not sure of the exact name) to activate the deadbolt on the inside while requiring the key for the outside?

@CWOTUS there may be several legitimate reasons to have keyed access to a room. I know I put a keyed lock and a deadbolt on the door going into the room that contains my gun safe.

But they make keyed doorknobs specifically for indoor use. You don’t have to go with outdoor hardware unless you want to.

@WestRiverrat I think that’s a perfectly sensible idea, and I wouldn’t gainsay that for a minute. I once worked with a man who told me – and to this day I don’t know if he was joking or not – who said that he had keyed locks on his kids’ bedroom doors, and that when they went to bed at night he locked them in. Maybe he had good reasons for that, or maybe he really was kidding, but I just couldn’t imagine it. Though as my own kids grew I thought about that a lot more.

@CWOTUS

And if you had, they probably would have gone out the window and shimmied down the drainpipe :)