Have you got a beautiful wife, a kid, and few dogs? I'm sure you do.
Yup, you know it, you need that mansion to keep them satisfied.
But before you can buy it, you need to become big in the property market.
Start off small, and keep selling at the right time to create your fortune.
It took me 21 years in this game before I was able to buy the mansion. The dogs didn't make it.. :(