Extending existing velux loft conversion

[YouknowmeIdowhatIdo]

We are thinking of buying a house with a recently converted loft but it’s within the existing roof structure and just has velux windows.

We want to extend the dormer (about 4m), and put in a bathroom and a bedroom. Has anyone done this? Any idea of costs? The existing conversion was done recently (5 years or so)

Is there precedence on the street? If so this is a good sign but I’d check the council planning portal to look for similar applications. Like everything else construction related, costs are going to vary wildly depending on your location.