When renovating it is a good idea to start with some existing drawings and your original site survey.
Your local council often hold prior development application documents in their records. When prompted, the council are able to retrieve these records.
You should contact your local council by phone or try and search their website for an Access to Information Request Form. Once you fill out this form council can retrieve the requested information and documents.
It can take up to 2 weeks or sometimes longer for the council to retrieve your documents.
When requesting document retrieval from archives you should explicitly state in writing that you want to retrieve any and all copies of plans, elevations, sections and any and all site surveys and titles (and prior letters of determination). You can also request your previously lodged engineering drawings and all planning and engineering reports.
Your architect will be able to speed up the design process by starting with these documents. Often however the architect will need to visit the site and perform a measured drawing process. This process allows the architect to confirm the accuracy of what has been built and that it matches the old drawings. Otherwise, future design iteration and planning may be erroneous due to redundant or inaccurate initial information.