As far as marketing strategies for single dog homes- I would first reach out if you haven't to local vet offices. Often times when a pet parent has had a pet die, they call their trusted vet first when they are ready to look again (I know because my day job is as a vet tech) and ask if they know of any dogs in need. Establishing a relationship with a vet clinic may mean that when this happens they will be more likely to tell them to contact your shelter first. Let's face it, for single dog homes only you are basically trying to catch pet lovers in between pets.
Some ideas for promoting: "One on One" with lots of pictures of one person and one dog looking very happy, "It Takes Two to Tango" (Two being one dog+ one human), "Three's Company" - A happy couple with one dog. For some reason I am hearing the song "Happy Together, thinking about these "campaigns".
Another thing to strongly consider- homes with out pets often are people that aren't sure that getting a pet is a great idea... maybe promote them for over night stays or day trips with people. If they form a bond with the dog they are more likely to decide they are ready to have one.
BUT- I am wondering how you determined that they can't be with other dogs? I know many dogs really can't, but if they are long time residents have you done any behavior modification to try to work on this problem? A lot of dogs are labeled single household dogs for liability because of one or two bad reactions and being safe IS best, but it may also be condemning them to long lonely lives in kennels. Just something to consider. If you haven't already, get a behaviorist to come meet some. You could tell the behaviorist that you will promote the heck out of them coming to visit and donating their time... which should be ample payment since rescue dogs often go to people who decide to get a behaviorist.
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