You have a few options:
You can disclose that information to your estate agent and intend to sell your property as is. The risk with this option is it will likely reduce your sale price.
Another option, is to get the addition approved retrospectively.
The resolution and certification of unauthorised building work can be a complex procedure and the time and cost associated with the approval of the works will depend heavily on the circumstances, documentation, timeframe, access and workmanship associated with the unauthorised works.
To determine the best approach, we’d recommend our professional staff undertake a Preliminary Inspection to determine the extent of works required – depending on the scale of the project, this will require a small investment of around $400.