If there is any ambiguity or question over whether your proposal passes the permitted development tests you have a number of options. It may, for instance, be possible to alter your plans to ensure they meet permitted development limits and conditions.
For peace of mind you may choose to apply for a lawful development certificate (LDC). This is not the same as planning permission but is proof that your household building work is lawful.
This option is well worth considering even if you are sure your project is permitted development. If you should later want to sell your property, an LDC may be helpful to answer queries raised by potential buyers or their legal representatives. As such, it is important that all paperwork and records relating to your property are clear and up to date.
You can apply to your local council for an LDC using the Planning Portal's secure online application service. You will have to pay a fee.