The best comments are those that relate to the planning framework. Some reasons you could use:

• Layout and density of the building (i.e. accommodation for too many people)
• Over-development of the site
• Loss of light or overshadowing (relevant for immediate neighbours)
• Overlooking / loss of privacy (relevant for immediate neighbours)
• Visual amenity (e.g. removing the chimneys)
• Adequacy of parking (relevant for those living in the same road)
• Traffic generation (relevant for those living in the same road)
• Noise and disturbance resulting from use
• Effect on listed building and conservation area
• Landscaping (loss of trees / any green garden)
• Design, appearance and materials

N.B. You can’t object on the basis that the development will reduce the value of your own property.