It is not possible for you to build a big house at the beginning, you cannot afford it. Some things must be postponed, like another floor or swimming-pool.
Making house bigger
By using a special tool you may move the walls of the house - the ones between rooms as well as the walls of the house. Without a serious mess you can make house bigger in a fast way. You can do it whenever you want because all objects move with the wall and walls and floors have the same pattern of wallpaper or tiles. To do it, move the arrow onto a wall until a green informative arrow appears - it informs it's possible , then after pressing left button on the mouse, move it in the direction you want to move your wall . Of course it is not for free.
You cam afford at the beginning, but it's not worth it. It will take up much space. After installing it a chimney will appear on the roof . Remember that in the future, it may make building new floors more difficult.
It is worth decorating the area around the house. Use trees  and of course settle them in such way that they won't stand in your way during the game. Think about a proper number of flowers, or bushes. You can also have a path  and change the outlook of the grass.
When you use the tool for creating swimming-pool you may build yourself one . Don't forget about the light and ladder. If you want to, you can change the covering of the wall and floor of the swimming-pool by using all available patterns from categories of walls and floors. In the future, when you can afford it, you can make it bigger, more beautiful and decorate the whole area .
Building a floor
When you have more money you can build a floor. Unfortunately, it requires much space at the bottom of the steps , so I recommend making upstairs bigger after making the ground floor bigger. You go to a higher level (click "Page Up"), and, the same as in the case of building a house on the ground, you start building walls and cover floor with something . Don't forget to check up on the roof and paint the inner walls as well, not only the outer ones.
Terraces and balconies
Creating a small balcony on a floor is easy - you just have to cover the floor outside the walls, put up some bars (as a low, light fence) and doors leading toward it. However, you won't make anything huge without support. That's why you need pillars, which hold the terrace built on the floor . If you built a house on foundations you can prolong them at the back to create a natural terrace at the ground floor with stairs leading to the garden.
Other elements around the house
Develop the area around the house - you can do it on many different ways (it depends on the size of the plot). You can create a pond with trees creating a small park. You can make a fence around the plot and make an entrance from the street's side. Next to the house, if you have children, you can build a playground for them . Don't forget about the drive for cars - will you built it outside , or in the garage - it doesn't matter.