Hi Tony,
As Freddy says, mainly for peace of mind, particularly if the person subsequently decides to sell their property. I've come across quite a lot of examples over the last few years where buyers have refused to proceed with a purchase unless the owner obtains an COL. This means a retrospective application, and a delay of up to 8 weeks (or more), which in the best case is stressful and in the worst case can cause the chain to collapse. At the end of the day, even if you're 100% sure it's PD, there's still a fair chance that a buyer will require a COL, particularly with significant works such as a loft conversion.
The other issue is that it's not just a case of being 100% sure of PD legislation, it's also a case of being 100% sure of the history of the property. I've come across examples where people have erected extensions without realising that their property doesn't benefit from PD rights - maybe it's subject to an Article 4 direction, or maybe it's had its PD rights removed by a condition on a previous planning permission. As a (hypothetical) example, suppose the property was deconverted from flats into a house in the 1980s via a PP where PD rights were removed, the current owner isn't aware of this previous planning permission, and the planning history on the LPA's website only goes back to the 1990s. In such a case, it would be next to impossible for an agent to realise that the property doesn't benefit from PD rights unless he submitted an application for an COL.
Of course, the above type of example is relatively rare - so I guess it all depends on what level of risk the applicant is willing to take.
Thanks,
Steve