My rear extension will include a new toilet – do I need planning?

Assuming that your rear extension is single-storey and you comply with the relevant limits, this work can proceed under Permitted Development. Note that side-facing windows are required to be obscure and the top opener must be higher than 1700 mm. Below ground drainage and mechanical ventilation are aspects that will require Building Regulations approval. You will be able to submit a building notice for this type of work. Ensure you get a completion certificate.