Almost always, yes. Mansard conversions alter the roofline in a way that falls outside Permitted Development rules. Full planning permission is required in the overwhelming majority of London boroughs and is always required in conservation areas. Simply Loft manages the full planning application on your behalf. The statutory determination period is 8 weeks, though most London boroughs take 10–13 weeks.
>> Do I need planning permission for a mansard loft conversion in London?
