There are several extensions that can be built without applying to the council for planning permission. Loft conversions, outbuildings, porch extensions, single storey and some double storey extensions would fall into this category. Even if your proposed works comply with the permitted development guidelines, I would always recommend applying to the council for a lawful development certificate, which is a similar process to house holder planning applications. You don’t have to go through this process but if you intend to sell your property, this certificate could avoid any delays in the sale or prevent any awkward conversations with neighbours who don’t fully understand what you can and can’t build under permitted development. It also acts as a peace of mind, as carrying out building work without planning permission can result in the Local Authority enforcing you take down extensions that don’t comply with all the rules. Permitted development is great and clear, especially if you live in a house that hasn’t been previously extended or is not an odd shape. Existing extensions or L shaped houses change what is allowed under permitted development, so before completing any building work I would recommend getting a second opinion from and architect, architectural designer or speak to the local council.