A council-approved extension does not automatically protect your home's value, as planners focus on planning policy and public amenity rather than private property devaluation. Whether the extension harms your property depends on specific factors: large structures close to your boundary—particularly two-storey or side extensions—are more likely to reduce value by blocking natural light, compromising privacy, or creating visual obstruction [1][2]. Small rear extensions typically have minimal impact. In some cases, a high-quality extension may even stabilize or increase local values by improving neighborhood standards [1]. The extent of harm varies widely; extreme cases suggest losses up to 10% [2], though most extensions have modest or negligible effects. You cannot sue your neighbor simply because property value drops, and planning objections based on devaluation are typically disregarded [2]. To assess your specific situation, consult a local estate agent or chartered surveyor for a professional valuation.