A SSAS can lend to a sponsoring employer or to a third party. A SIPP can only lend to a third party. Loans to members or those connected to a member are not allowed. The loan must not be granted for more than 5 years, but under certain circumstances the loan can be extended by up to 5 years.
HMRC Pensions Tax Manual - PTM123000: loans: contents
HMRC Pensions Tax Manual - PTM123200: Term of loan