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In STH there are only two extensions - the Dog House and the Kitchen. These maybe be the only ones but they're well worth it.
 Dog House
You can't get a dog house until you have caught your dog. The dog house is good, because once you get it, you don't need to bring your dog inside when it rains. It will just go into its dog house and be safe and dry.
This is a very wise investment. You can cook all sorts of food using the frying pan, pot and oven. It also comes with two extra rooms. The one on the right is a toilet, and the one on the left is a bath. Use the bath to restore your stamina. It also comes with a fridge where you can store foods.