What happens if I cast Leomund's Tiny Hut in a space too small to contain the dome?

Question

Leomund's tiny hut creates a dome-shaped barrier with a radius of 10 feet. This means that the dome is 20 feet wide in all directions, and the top of the dome is 10 feet above ground level.

Suppose I am a traveling wizard who feels that the inn where I am renting a room does not provide adequate security, and I'd feel much safer if I could put up my tiny hut in my room before going to bed. Unfortunately, the room is only 15 feet wide and the ceiling is 8 feet high, which means my tiny hut is not quite tiny enough for the room.

What happens when I attempt to cast Leomund's tiny hut in this enclosed space? Does the spell fail? Does the barrier extend through the walls, possibly inconveniencing the people in neighboring rooms? Or does something else happen?

Answer 1

It does not penetrate the walls.

The PHB p.204 talks about areas of effect:

A spell's effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area.

The point of origin is yourself and it's a ten foot hemisphere, so the area that becomes the hut is from you, out to a radius of ten feet, constrained by the walls of the room.

The spell doesn't specify whether the reduced area would also reduce the number of creatures you can contain in it, so that's up to the DM (though since a ten foot radius centered in a five-foot square would include nine five-foot squares and a number of fractional squares, it'd be reasonable to say the reduced-area hut can contain as many creatures as full five-foot squares).