You don't have to be an architect or designer to produce plans and submit for a Planning application. However - Planning authorities will expect a certain standard of drawing - which accurately describes the proposal. Also - a Planning officer is likely to ask questions about your proposals during the application process. Architects and designers possess experience and knowledge which enables them to respond appropriately. They will also provide the right advice about matters such as space planning, proportions and materials - which often form part of a design scheme and Planning application. Therefore - depending on your skills and knowledge - it may be worth the cost of help from an architect or designer.

It depends on several things, if the extension is complex then almost certainly yes you will need a architect for planning. The reason is planning will need size style and material specification including roof styles and heights- if you get it wrong then you start the process again. If it’s a simple single storey extension then you may get away with doing it yourself.
Do not get confused with detail for planning and building regulations, they are 2 separate drawings BR are more detailed showing steel( if required)

You could use a architectural technician

In answer to the question can anyone produce Planning drawings the answer is
probably yes they can. However why do Architects spend several years in training at University if the task ahead of them was so simple? The answer is you get what you pay for as with everything. An Architect offers a wealth of experience not just drawings all of which can benefit a Client. An Architect has full understanding of geometry, proportion, materials, Building best practice, Planning procedures, time frame and budget. This knowledge gives best results.
Being an Architect is not just about producing drawings it is about far more.

You do not need to be an architect, you can appoint a design professional who is qualified and experience in carrying out architects services such as a chartered architectural technologist who in many instances will be able to provide you with detailed planning and building control informaiton an architectural technologist specialise in design, underpinned by building science, engineering and technology applied to architecture within projects, playing a pivotal role in project and design management.