What happens If you  personally owns a number of buy-to-let residential properties and decided  for a member of the family to live in one of those let properties. Could the cost of repairs made to that property while they are living there be claimed as expenses against the rental income from the whole property portfolio? 

Any expenditure on repairing a property will be incurred for the benefit of your client and/or their family while they live there, so it can’t be deducted from the profits of their property letting business. Even if the property is let out again after their family has moved out, the repairs done while the family were there have a non-business purpose. Therefore, it would be better to do the repairs while the property is occupied by a tenant or after their family has moved out.