Some of the House Additions are pretty important in this game. Others just seem to have no purpose whatsover.
Price- 1000 G
The Baby Bed is neccessary to having a Baby. The Baby sleeps in it and is built next to your Bed.
Price- 3000 G
There are three things in the Bathroom. There's a Sink which has no purpose. There's an unseen Toilet where you can here Jack "relieving himslef." There is also a Spa which cures you if you are sick. That's the main use of the Bathroom.
Price- 30,000 G
The Greenhouse is the BEST House Addition in the game. If you have it, you can harvest all of the Crops all year long inside of it. The only Crops that can't be harvested all year round are the Strawberries. Those can only be grown in the Winter. Grass and Flowers can't be planted in the Greenhouse either. Time stops inside of the Greenhouse, so you can take all the time you want. Beware in the Summer. Typhoons can blow your Greenhouse away.
Price- 5000 G
The Kitchen is neccessary to getting married. Rick won't sell the Blue Feather without it. In the Kitchen, there is a Freezer which can hold food items and keep them fresh. There is also Recipe List which shows all of the Recipes you've collected. There is a Power Berry List which shows where you've collected each of your Power Berries. Also, if you come home at 6:00 P.M. on important dates like your Birthday, your Wife's Birthday, your Anniversary, or your Child's Birthday, your Wife will have prepared a feast. She'll then ask you why the date is important, so keep track of the things listed above.
Price- 7000 G
There is no purpose to the Log Terrace. It just sits behind your House. On Party Day, you'll find some of the Party goers here.
Price- 1500 G
The Stairs lead to your Roof. You'll find your Wife here occasionally.