Best way to fill an odd-sized gap?

Hi, I trying to find the best way to repair a flaw in my design. I accidently left an opening and would like it to just become a flat surface, perfectly level with the other geometry. Ideally I’d just extrude the portion I marked on the picture, but that’s of course not really possible. I tried to create a box and unionized the two objects, but it never gets really flush if you zoom in far enough and the added geometry seems like a mess. Is there a cleaner way of filling that gap or is that the only way to go?

Use split body tool to divide floor into two elements at the face of the jamb.Then extrude triangle to split floor face. It will stop where the floor has been split. Use the union tool to recombine the two split floor pieces.

That worked wonders, thank you.