If I could design my house any way I liked, it would most certainly include:
1. Secret passages!
2. A cupboard under the stairs large enough to hide in
3. A treehouse in the back yard, which would be fully weatherproof and contain console gaming systems and beanbag chairs
4. A room that included a loft-type thing or a platform with a ladder leading up to it, and a pile of pillows, beanbags, etc., on the floor that you could jump off the platform and land in
5. A four-poster bed with curtains
6. A kitten!