Many private lenders invest from a self-directed IRA or 401K. To be able to make any kind of self-directed investment you need an IRA with a self-directed IRA custodian, through an industry leader such as Equity Trust.
IRA Plans with checkbook control
The best structure by far is a self-directed IRA Plan with "checkbook control", offered through industry leaders like IRA Financial (where the modest annual service fee includes attorney support). IRA plans with checkbook control are easier to manage (no bureaucracies), and the annual cost is much lower (no transaction or processing fees). Recommended!
Flexible investment amounts
We can often accommodate investments for less than the full amount needed with a pari-passu structure. In this case multiple investors fund only part of the funds needed, and still receive first mortgage security. Useful for allocation of otherwise idle (IRA) funds.