You should organize dog meetings on a neutral territory

First of all, if you want to add a new dog to your house, you have to organize a dog meeting in a neutral space away from your house where one of them already feels territorial. After this, the next step is to keep a safe distance between them. 

“It is best to introduce dogs on neutral territory away from the home in a relaxed environment. Whether your dog is new to you or not, a good way to set up a dog meet and greet is to go for a walk where they can stroll near each other,” said Sydney Bartson Queen, an animal behavior counselor on the ASPCA’s behavioral sciences team.

When your dogs become more comfortable with the situation per se, you can gradually minimize the distance between them and encourage them to get to know each other better. They can sniff each other, but you should be cautious. 

“As the dogs become more comfortable, you can progressively walk them closer and closer together,” Queen says. “Keep a close eye on both dogs to ensure neither dog appears fearful (low body, mouth closed, ears pinned back, looking or moving away from the other dog) or threatening (stiff body, tail high, stiff or fast tail wag, closed mouth, tense face or eyes).”