I do some commercial work but not a huge amount. Several shops, a hotel, a few care homes, a couple of offices. Most of it is subcontracted from main contractors though. My route in has been varied. I picked a bit of it up via Yell/Yellow Pages when I had a bigger ad in there. A bit of it came from window cleaners who I know who still do things the traditional way so weren't able to supply the service. Some of it comes from a couple of cleaning companies. I just happened to be in the right place at the right time (I cleaned a house next door to where he lives). So I think much of it has come through just being around for a long time and having a trustworthy name (i.e. not going behind the contractor's back to make deals with their customers). Some subcontract work is, by its nature, a bit tight on price but this can be made up if it's the right sort of work. With the jobs I do for one of the cleaning companies, some of the prices are a bit grim and some are pretty good. Therefore, rather than trying to cherry pick the best work, I take the view that the work pays fairly OK on average so I put up with the poorer work so that I can also do the better work. Many of these are the sort of jobs I may have had trouble gaining access to just by offering a window cleaning only service.
I have at times considered starting up in general cleaning but after seeing some of the aggravation involved (as a part-time employee for a while), I decided it wasn't for me.
I didn't canvass or seek out any of the commercial jobs. They came to me. If I had sought them out I'm pretty sure I would have more of it.
Having a fair chunk of my income via subcontract is potentially a vulnerable position. However, I take the view that even if one or two sources dry up, the others ought to be OK. Also, I exchanged a lower rate in the here and now for canvassing my own, earning more, but with a delay in getting paid. My current circumstances make this my better option for now.