Have you noticed  

unkept homes usually have unkept animals? 

When I first began spay/neuter outreach these homes were the first I visited.
I knocked on the door, introduced myself and asked if we could help fix their pets. If you tell folks it makes their life easier, most people welcome the help!
We offered free spay/neuter and transport. When we transport, it eliminates excuses. If possible, have the pet owner pick up their pet. It creates investment and educates them to using a vet clinic. Unfortunately, there will usually be more pets at these homes... so teach people responsible care. It's  long term gain!