If you are self-employed, you can set up a solo 401(k). The $18,000 you refer to is the limit for elective employee deferrals. You can contribute 25% of your self-employment compensation (20% if you're a sole proprietor or have a single-member LLC) as an employer profit sharing contribution. In total, between employee and employer contributions, you can contribute a maximum of $53,000/year.

The great thing about a self-directed solo 401(k) is that you can purchase physical real estate in the retirement (speak to qualified counsel about the rules). Also, if you want to buy a home, you can borrow up to half of your 401(k) balance (to use as a down payment) and, if your plan allows, the repayment term for a loan used to purchase a primary residence can typically be extended beyond five years (the typical term).