If you are in the UK, it is worth looking into a buy to let mortgage for potential additional income. Buy-to-Let mortgages are becoming increasingly popular in the UK.
For those not familiar with the term, this type of mortgage is for anyone interested in purchasing a property to let (rent) out. The upside is the extra income the owner can make from renters and the potential additional profit if the house increases in value when it is sold at a later date. Overall, it is a low risk investment if the buyer does their homework on the property.
The downside is the risk of not being able to let (rent) the property out for enough to cover the mortgage. Another potential issue is the house decreasing in value and the owner losing money when/if they sell the home.
If the buyer does the appropriate research on the property and its surrounding market, considers what being a landlord entails (if this is their first experience), and the also looks at specific details of the buy to let mortgage, the investment should be a profitable and satisfactory experience. It is also important to note to be patient along with being thorough. This is after all, a long-term investment.