Property empire

My husband and wife clients own a retail shop which has traded successfully for quite a few years. They have recently purchased another shop on the other side of town originally planning on carrying on the same trade there and hopefully expanding their customer base. The new property will require some updating; installation of fire safety equipment fire escape heating/hot water systems etc. The upper floors of the property have previously been used as an office but someone has mentioned 'flat conversion' to them and they are now wondering whether to use the downstairs shop and let out the upper floors as one or two flats.
Can readers explain briefly how the flat conversion allowance works what qualifies and how it is used? Also if some expenditure — as mentioned above — relates to the whole property are there any advantages...