An extension does not need to meet standards specifically to avoid property devaluation, because under UK planning law, potential devaluation of a neighbouring property is explicitly not a material consideration [1]. However, planning authorities do assess extensions against regulatory standards designed to protect residential amenity. These include the 45-degree rule to prevent blocking daylight or sunlight to neighbouring windows, minimum separation distances between facing windows to prevent overlooking, proportionate scale and massing relative to the original building and streetscape, and compliance with local design guides regarding materials and character [1][2][3]. Extensions must also comply with building regulations for structural stability, fire safety, and energy efficiency [2]. While meeting these standards ensures planning permission is granted and reduces factors that might negatively affect market perception, the council's legal duty is to protect amenity, not property values. Market fluctuations and subjective preferences remain outside the council's remit.