You can purchase a Commercial Property within a SIPP. The rental income received is tax free as it is invested into the pension, and there is no capital gains tax to pay on the sale of the property in the future.
You must remember that the pension scheme owns the property, not you individually. Therefore costs relating to the property must be paid from the pension scheme and not by you individually. Rent must also be paid at market rate for the property, therefore if you are purchasing your own commercial premises for your business through the use of a SIPP, you must still pay rent for the property. The rental income does not count as a pension contribution, it is classed as a return on investment and so you still have your full annual allowance available for pension contributions if you are receiving rent from the property.