I want to put in a smaller pet door. How do I fill the hole from the old one? Putting in a new pet door that’s smaller than the old one can be a little tricky, but it’s definitely manageable. The best option will depend on the situation. If the new door is just a little smaller, it might be possible to fill out the gap with some wood and cover with a molding that fits the overall look. If the pet door is installed in a household door, the easiest might be to replace the entire door. If it’s installed in a wall, options will depend on the size difference and type of wall. Give us a call at 800-749-9609 and speak with customer service. They have extensive experience and will be able to provide good ideas.