Check whether an extension has planning permission by searching the local authority's planning register for applications relating to the property, since extensions are often logged separately from the original house permission. If no application appears, the extension may either be exempted development under the Planning and Development Regulations 2001 or unauthorised, and this should be confirmed with the local authority.
Extensions are usually recorded as separate planning applications from the original dwelling, so a search for the property address should be reviewed for all entries, not just the earliest one relating to the house itself. The decision documents will show the scope of works approved, including floor area, height, and any conditions, which can then be compared against what has actually been built on site.
Not every extension needs planning permission. Many small, single-storey rear extensions fall under exempted development limits set out in the Planning and Development Regulations 2001 (as amended), Schedule 2, provided they meet specific size, height and boundary conditions. If no planning application appears on the register, this does not automatically mean the extension is unauthorised, it may qualify as exempted development, but the specific conditions should be checked carefully, ideally with the local authority or a professional.
Where an extension exceeds exemption limits and no permission was granted, it is unauthorised development, which can affect mortgage approval, insurance, and future sale of the property. In some circumstances, retrospective (retention) permission may be sought, though this is not guaranteed and depends on planning policy at the time of application.