Lou, it's always good to have a "plan"
Doggy Doors (IMHO) are invaluable, but, there are some very well
informed and concerned Users here, who have mentioned situations
which could give one pause.
If you, or anyone would like to read the "discussion" on those issues...
http://www.yorkietalk.com/forums/yorkie-products-reviews/224269-doggy-door-safety-convenience.html All in all, being informed is the best way to go.