Neighbour impact is one of the most common reasons for planning refusal. Councils assess whether your extension will cause unacceptable loss of light, create an overbearing sense of enclosure, or reduce privacy through overlooking. Even well-designed extensions can be refused if they significantly harm your neighbour's living conditions. The impact on immediate neighbours carries more weight than objections from properties further away.

The 45-degree rule is a guideline used to assess whether your extension will block light to neighbouring windows. Draw an imaginary line at 45 degrees from the centre of your neighbour's nearest ground floor window. If your extension crosses this line, it may cause unacceptable loss of light. While not a legal requirement, many councils apply this test to rear and side extensions, particularly on terraced and semi-detached properties.

Loss of light refers to a reduction in daylight or sunlight reaching your neighbour's windows or garden. Councils consider the size and position of your extension, which windows are affected, and what rooms those windows serve. Kitchens and living rooms receive more protection than bathrooms or hallways. Some councils use technical assessments like the BRE daylight and sunlight guidelines for borderline cases.

Overbearing impact describes when an extension creates an oppressive sense of enclosure for neighbours. Even if light levels remain acceptable, a large structure close to the boundary can dominate a neighbour's outlook and make their garden or rooms feel boxed in. Councils consider the height, depth, and proximity of your extension when assessing whether the impact is acceptable.

Neighbours can object but cannot automatically stop your application. Planning officers assess applications against policies, not popularity. However, if neighbours raise valid material planning concerns like loss of light, overlooking, or overbearing impact, officers must consider these. Strong objections backed by evidence of genuine harm carry more weight than general complaints about disruption or property values.

Overlooking occurs when windows in your extension allow views into neighbouring properties, reducing their privacy. First-floor windows and raised terraces create the most concern because they offer elevated sightlines into gardens and rooms. Councils typically expect 10 to 12 metres between facing windows and 21 metres between rear elevations. Obscure glazing or high-level windows can help address overlooking concerns.

For standard permitted development, no notification is required. For planning applications, your council notifies neighbours as part of the consultation process. Under the Larger Home Extension scheme, you must notify the council, who then inform adjoining neighbours and give them 21 days to comment. The Party Wall Act is separate from planning and requires you to serve notice if building on or near a shared boundary.

Under permitted development, you can build up to the boundary, but if you're within 2 metres, eaves height is limited to 3 metres. With planning permission, there's no fixed distance, but building closer to boundaries increases scrutiny of neighbour impact. On terraced and semi-detached properties, the depth of your extension at the shared boundary is often the most contentious dimension.

Your neighbour's existing extension doesn't grant you automatic rights to match it, but it does establish local precedent. If their extension was approved through planning permission, it suggests the council may accept similar proposals. However, every application is assessed individually. Your extension might affect a different neighbour or have different impacts, so the previous approval isn't a guarantee.

Keep your extension as far from boundaries as practical. Reduce height at the boundary by using a pitched roof sloping away from neighbours. Avoid windows that overlook neighbouring gardens or rooms. Consider the 45-degree rule when deciding depth. Use materials and designs that feel less imposing. Talk to your neighbours early to understand their concerns and adjust your plans before applying.