Not read all the comments but for me, one off cleans are great for adding extra money to your week in the nicer weather.
Only so much work you can do in 7ish hours work during the winter. That's if you're lucky not to get rained off for the day.
I don't pack our rounds to bursting point, falling behind on schedule, we're always on time and as such have a great reputation.
Add ons are great, I price them above what our average hourly rates on windows are too.