Under the 2004 Premises Costs Directions, income from private patients or 'under arrangements with any person who is not a public authority', a deduction will made to your reimbursement if this is 10 per cent or more of total gross practice income.
There is a table in paragraph 49 of the directions that tells the PCT by how much it should reduce the amount of any premises payment to you. This does not apply to any rent received from the separate subletting or renting of part of your premises, and paragraph 48 notes that the PCT must set off the amount you receive against the notional rent it pays to you.
While you are unlikely to be able to rent out part of your accommodation without affecting the notional rent, you can charge a service cost to cover such elements as decoration, repairs, utilities and so on. This should not affect on your notional rent. Download the directions at www.dh.gov.uk