In Harvest Moon 64, there are several extensions you can buy for your house to get a larger house.
1. Kitchen: Cost- 5000 gold Lumber- 450 pieces The kitchen is one of the three extensions with significant use. Besides great decorations, the kitchen has two lists in it. One keeps track of the powerberries. The second keeps track of the recipes you have gotten. Another feature the kitchen has is a freezer, which will let you store food and crops in it. Another thing is, if married, your wife will make you dinner if you enter the kitchen around six p.m. on special days (your birthday, anniversary, ect).
2. Greenhouse: Cost- 30000 gold Lumber- 580 pieces While the greenhouse is the most expensive and lumber consuming addition, it is the most useful in the game. It goes in the small yard area south of your house. The greenhouse has many features, such as allowing you to grow crops in any season (including winter), grow out of season crops (spring crops in fall), grow strawberries, it has a spot to fill your watering can, and it's own shipping bin. Even better, time stops inside the greenhouse, so you may take as long as you want planting, watering, and harvesting your crops. The downside is the greenhouse can be blown away during the summer if you have a storm.
3. Bathroom: Cost- 3000 gold Lumber- 300 pieces The bathroom has little purpose, but is a fun addition. It has two parts. First, a toilet, which is just for looks, and a hot tub which will help keep you from getting sick. Contrary to belief, it does not restore stamina.
4. Baby Bed: Cost- 1000 gold Lumber- 200 pieces The baby bed is, well, a bed for the baby. If you intend to have a baby on this game, you need this.
5. Staircase: Cost- 2000 gold Lumber- 250 pieces Another useless extension you can buy, but it does serve as a way to get on your roof. If bought, it puts a staircase to the very top right of your main room in the house, and leads to the roof.
6. Log Terrace: Cost- 7000 gold Lumber- 350 pieces The terrace is basically a little patio out behind your house. It contains a small picnic table, and serves no use until you beat the game (your dad comes to check the farm).