Most people are aware of the existence of planning and building regulations but, in my experience, many people (even some in the industry) are unsure of the differences between them and have little knowledge of the processes involved.
The first main difference is what they control. Planning legislation is principally concerned with making places attractive, sustainable, and convenient. Planning governs your right to develop, and influences the design and appearance of development, and how it may affect other people, eg privacy, outlook, etc. In terms of a residential extension, the design would generally be assessed on how it would affect the character and appearance of the existing house and its locality, if the materials would be visually acceptable, whether there would be sufficient parking, and how it would affect neighbour amenity.
Building regulations are concerned with ensuring health, safety, welfare and convenience of building users, and with energy conservation. They consist of a set of functional requirements, covering areas such as structure, fire safety, drainage, ventilation, etc, which are backed up by technical guidance in Approved Documents which show ways of complying. This is generally much less subjective than the planning system, as following the guidance will usually ensure approval.
Another key difference is that the planning system is publicly accessible, whereas applications to Building Control are not. So, when a planning application is submitted, it is published online by the Local Authority, who also usually post site notices to alert neighbours and the general public, as anyone can comment on a planning application. Some applications, mainly involving listed buildings and conservation areas, still have to be advertised in local newspapers.
Some projects do not need planning permission or building regulations approval, and there is often a considerable amount of confusion about this. The General Permitted Development Order (GPDO) sets out work that does not need planning permission, and is therefore Permitted Development (PD). It is quite complicated, which is why there is a 51 page official technical guidance document showing how to interpret the rules. Building regulations exemptions are less complicated, but there are overlaps between the two sets of legislation, which can cause extra confusion.
Conservatories are a great example. They are usually exempt under building regulations, subject to a 30m2 floor area limit, and a requirement that they are thermally separated from the house (ie by retaining the existing external doors/windows). However, they are treated the same as "normal" extensions under planning, so some will be PD, others will need permission - it all depends on certain key dimensions and where they are located. There is a lot of useful information on the Planning Portal website, which is probably the best starting point, but professional advice is always recommended.
Please feel free to comment below, or contact McAndrew Associates directly if you need assistance negotiating your way through the minefield!