If you have to make it a separate page that’s available by visiting a permalink, you have to think what happens when user visits this page directly, without visiting the single post (house) first.
If you know how to handle it, the easiest way would be to make the reservations.php it’s own page template and just link to that page from within your single post, while adding post ID as an argument.
The best way to do this, however, is, like kaiser suggested, include the reservation form in the single post template.