There are 6 different home extentions you can get. You get them from the Carpenter's House in the mountain(across the river). They all cost a certain amount of money and wood. 

Name Money Wood Description 
Kitchen 5,000 450 The Kitchen adds a large room to the side of your house. In it is a table, refridgerator and some counters. You can't cook, but you can store things(like produce) in the fridge. You can also on the wall see how many power berries you have and the recipes you have! 
Bathroom 3,000 300 The bathroom is added to the Kitchen, or if you don't have the Kitchen you have to go outside then go in the door to the bathroom. It comes with a toilet(can't see it but it's in the little room) and a bath. The bath helps with Stamina, I guess the toilet does too! 
Stairs 1,500 150 The stairs lead up to the roof where you can walk around. There's no real purpose maybe they were going to have a second story but if you want to get all the photos you must build stairs! 
Deck 7,000 350 The deck goes behind the house. It is actually very large. It will have a purpose when your dad comes. 
Baby Bed 1,000 200 You need the baby bed to have a baby. It is just a small bed that goes at the foot of your bed, and when you have a baby that's where it sleeps! 
Greenhouse 30,000 580 The greenhouse is VERY useful. You get a very large space, and you can grow any type of crop you want all year long! This is good for winter when business is slow. VERY good investment! It might be blown over in hurricanes but might not!