36 Mortimer Avenue
Graham and Grace O'Brien lived together in a house in Sheffield with Grace's grandson, Ryan Sinclair. The house was number 36 on its street. (TV: Arachnids in the UK)
When they first met the recently-regenerated Thirteenth Doctor, and she fell unconscious, they took her to their home. When the Doctor woke up from her nap, she complimented them on their sofa. (TV: The Woman Who Fell to Earth)
Following Grace's death, Graham and Ryan continued to stay there when not travelling with the Thirteenth Doctor. Graham actively chose to travel in the TARDIS, as his house was haunted by memories of his loss, and he needed time to process his grief. (TV: Arachnids in the UK)
Team TARDIS returned to water Graham's houseplants, a few weeks after they had left in Earth time. (PROSE: The Secret in Vault 13)
Behind the scenes
In the real world, the Sheffield house which stands in for Graham's house is located at 36 Shrewsbury Road, just outside the entrance to the Park Hill flats. In Arachnids in the UK, Ryan says to Yaz "Wait you live at Park Hill? We're just up there", implying that the house is just up the road; this lends credence to Graham's house being located in the same place within the DWU.
Location information
Though it's possible that a real world location doesn't exist in the same geographic space in the Doctor Who universe, such cases are few and far between. Thus, the map to the right is probably a good indicator of the DWU location of 36 Mortimer Avenue.
