If you are planning to convert your existing basement without having to excavate, or change the external appearance of the property then it is not likely that you will need planning permission. However, if you are planning to retrofit your existing basement, by adding light, ventilation or additional structural elements then you will need planning permission. This of course applies to creating a whole new basement all together, planning permission amongst other mandatory SPD (Supplementary Planning Documents) will be required. You should always check with your local planning authority to see what information they require, as there are a lot of surveys and assessments that need to be carried out to determine the viability of basement construction, so it is always best to be prepared.