Adorable first home! I haven't downloaded it yet but I will.
I see you used all base game stuff, which is a great way to go. Not everybody has a ton of store or EP stuff. Since this is your first house, allow me to make some suggestions.
First, you can use the hand tool to grab walls and make the house just a little smaller. This will give you some more yard/garden space and will close up some of the space you were having difficulty using up. Also, you can make the roof line a little lower.
Use double wide stairs going upstairs to help prevent traffic congestion and to take up a bit of that space.
More windows would make the house brighter and more airy feeling inside. As a housewife, I really hate it when there are no windows in my kitchen. Nothing more depressing than having to stand over a sink of dirty dishes and that's all you can see. lol
Plants are your friend when it comes to decorating and filling up space. They also give a cozy feel to any room.
Have you considered moving the upstairs bedrooms around a bit? Forinstance, if you put two, slightly larger rooms at opposite sides of the building with a bathroom between them? If you pull the bedroom walls down toward the stairs so there are only three squares of floor left in front of those rooms, and make it a smooth wall all the way down, with two doors, one for each bedroom, you can then leave the half walls out and instead either leave this as a big recreation/media/play space. The middle section between the bedrooms, where you would put two, smaller bathrooms, could also have a door into that bathroom, or you could add a third, more public half bath. It will make the bedrooms larger but the space in the bedrooms is easily taken up with perhaps a bookshelf, chair, and floor lamp for some quiet time reading. In the "kids" room you could add another bed so two could sleep there and you could add a table and chair for doing homework. (Not that they would! My Sims kids always pick the kitchen or dining room table.)
To give the house a more welcoming, "curb appeal", add a walkway to the door and line it with some flowers. You can use terrain paint to make the walkway or any of the masonry/stone/tile flooring. Do a driveway in front of the garage as well. Again, terrain paint works just fine.
I like the way you made a bay window effect in the front of the house. Not just a box shape but something with some eye appeal. If this were my house in real life I'd have window seats in front of those, nice, big, sunny windows upstairs! In Sims life I think I'd put two love seats there on either side, leaving the center section for a coffee table. Add a floor lamp or two and voila, a cozy place to sit and knit while you watch the kids play in the large, open area.
Behind the garage, add some terrain paint "stone" or "brick" to make a patio, and how about a door out to that spot from the kitchen?
I don't actually build that much. What I do is remodel what is already there. Can you tell? lol
Don't be discouraged by any of my comments. For a first time this was a really good house and we all had our first houses.
