The reason for refusal here is By reason of the design and siting the erection of a single storey rear extension would bean inappropriate addition to the host dwelling as it is not of a sympathetic design or form. In addition, the proposed would result in harm to the visual appearance and character of the host dwelling, existing street scene and Conservation Area.
This appears to be a proposed addition to a recently constructed house, in traditional material in a small tasteful development. Unfortunately, the proposed scheme is very average, maybe the designers are trying to add a modern intervention, which is perfectly acceptable in this location, but the drawings do not portray this.
PAPA Says - In sensitive locations such as this, the scheme has to be better quality and better presented, this is an example of where a creative Architect should be employed to take forward the design.