I also create photogrammetry models (and other data) from terrestrial and aerial surveys of historic buildings as a building designer (via Pix4D). SketchUp is not suited (nor designed) to handle large poly models such as these types of software create, you would need a traditional modeller for that (c4D, 3DS, Modo etc). Also the mesh topography is not ideal.
However, I obviously use SU as my primary design tool and Pix4D as part of the workflow. I use my surveys and outputs from Pix4D as follows.
Orthomosiac as an accurate site and footprint plan, which I import into SU and trace to create the site footprint and boundaries.
Generate a mesh from the survey point-cloud (terrestrial and aerial) for my own use in terms of measurement and reference.
Generate contours from the DTM to bring into SU TIN.
I would have done a traditional survey alongside the aerial surveys and built My SU model from that.
For particular architectural details or intricate sections that I need to record, highlight and detail I use a Structure Sensor to capture the detail in 3D, which I do bring into SU and create an SU model using the imported mesh as a reference object, its managable at that scale but once if created my model I get rid of the reference. (see attachment)
SU certainly is an powerful and accessible tool for designing, detailing and presenting the existing building and the extensions repairs. I use it for all my listed building projects but I get build my survey dataset from a number of sources, aerial, traditional measurement, 3D scans etc,. The Photogrammetry model is fantastic for me to be able to understand complicated roof structures particularly where there have been several phases. Its also excellent for showing to clients so they can get an understanding of their building too.