A great many extensions and conversions to houses are carried out without professional help. We suspect that builders carrying on business in this area actively discourage people from concerning themselves with details such as planning or building regulations, with the result that many are carried out without the necessary approvals and often without the assistance of an architect or engineer.
The result is that unauthorised extensions and conversions can be a source of considerable trouble on the sale of the property when they and their compliance with planning, building bye-laws or building regulations get scrutinised by professionals, often for the first time.
A distinction should be drawn here between problems with the actual building and problems with the paperwork, such as lack of the usual paperwork certifying compliance with planning permission and building regulations.
Problems with the building can vary from the worst situation (where the extension is so hopeless that the best thing to do is to demolish it completely), to the other end of the spectrum, where the faults are minor and can be rectified at little expense.
Problems with the paperwork can usually be overcome if the building itself stands up to scrutiny. But, again, there is a cost factor in getting an architect to certify the works or perhaps to apply for planning permission to retain an extension.