SIPPs have the ability to invest into a wide range of investments just like the standard personal pension, but it also can invest directly into commercial property.
There is a limit as to how much an occupational scheme (ie a SSAS) can hold:
under 5% of scheme assets in any one sponsoring employer; and
under 20% of scheme assets, where the shareholdings relate to more than one sponsoring employer. However, the shares in any one sponsoring employer of the scheme must still be less than 5%.
The value is only calculated at the time of purchase and no re-tested until a further purchase is made
There is no restriction on the percentage of shares held in one company (so you could own 100% of a company as long as it is below 5% of scheme assets)