For example could I use current cash in a SEP retirement account to buy real estate--a house for personal use?

Real Estate can be included within an IRA however there are many caveats to be aware of and you have to use a trustee that is willing to do that. You cannot use outside funds to improve the property unless they qualify as a contribution.

http://www.penscotrust.com/ is a firm that handles alternative investing with an IRA

I was intrigued by your question, although I'm not sure I fully understand it. Are you talking about buying real-estate and then placing the deed into a retirement account? Or, are you considering withdrawing money from your retirement account, buying real-estate, then placing the deed in the retirement account?

I did find this article. You should check it out. Might help. http://www.filife.com/stories/using-real-estate-to-build-your-individual-retirement-account