What is the purpose of the door? What problem do you need to solve?

We air bnb the lower level and want to put in a door that is lockable and blocks noise.

The door must swing into the room, not over the stairs.

Straightforward pre-hung door into new framing at the top and sides.

Put the door at the bottom of the stairs.

Added image of bottom of stairs. Had heard bottom of stairs would not be an option for not enough landing space or similar. Would def. prefer bottom if an option though.

No pic. When we rented out two apartments in a walkout basement, we had doors top and bottom. One or two, the door(s) should be solid core and tightly fitted to impede sound transfer.

You don’t add a door to either. It isn’t safe.

Is there another way to close off the downstairs unit? Like, is there a little hall or something, so the door doesn’t have to be at the stairs?

I’m not sure about codes, but can you drywall either at the bottom or top? Isn’t it a separate unit, with its own entrande?

Thats the bottim if the stairs, there is a landing, could a door go there?

Lppoing at the last image, a double door opening into the landing seems possible. The width of the opening and the return dimension on each side is necessary. The opening on the left isn't fully shown. That opening width might be decreased to add depth to the landing. The wall on the right could be brought forward to match. Exterior door weatherstripping, including a bottom door seal would reduce sound transmission.

The top space has an industrial look. A rolldown door could fit the decor. No swing problems. It could be powered. 3x7' mini storage door. 3 x 7 Roll Up Door Model 650 Order Configuration (rollupdoorsdirect.com) 23 colors. Can be insulated.

How is a door at the top of the stairs not safe? Every house I've had has a door to the basement.

How is a door at the top of the stairs not safe?

Only if it opens towards the stairs and there's no top landing; anyone going up the stairs could be knocked down the stairs by a door suddenly opening. Prohibited by Codes. (Unless one is in the benighted backwoods outside of all Code enforcement.)

The OP should be aware that the stairs may be a required egress for fire safety for a basement unit.

Oh, yes, now I understand. Most homes with stairs going down to the basement have doors that open AWAY from the stairs. The "landing" is little more than a step and a half.

In many houses the basement stairs, unless there is a full landing at the top of the stairs, the ceiling above the stairs drops down sharply enough that the door could not swing into the stairwell anyway, if it is a wide door.

Actually at the bottom of the stairs there should be a three foot landing as well, if the stairs are three feet wide. (I think you need the same depth of landing in the direction of travel as the width of the stairs.

But this situation clearly needs door/s if the lower level is rented. I would probably want them top and bottom.

You might be able to do a barn door, but if your problem is toddlers, they'll grow remarkably quickly and you won't need the door.

Does the lower level have its own entrance elsewhere? Or do your AirBnBers enter through your living space?