The amount of time it will take to complete your project will be determined by various factors such as:
The size of the lot/space
The scope of the renovation or addition
The permits you need
What you’re using the space for
A good design consultant will take everything into consideration before they start your project or even quote you.
In general, most delays in building additions are caused by permit delays. If you don’t provide  the correct drawings for a certain aspect of the project, this will delay your building permit. The most common mistake that homeowners make that seriously affects the timeline is that they hire professionals that will handle only one aspect of their project (an architect who provides only structural drawings and not electrical or other mechanical drawings, for example). That’s why you should always speak to design consultants who will provide you with a full set of drawings and will make sure they comply with the city’s bylaws.