Dan,
I started off just doing windows (water fed pole system) Got a stack of commercial work, along with domestic just from my website. Did not offer any other service for 2 years. Then I got asked about carpets, and other avenues of cleaning.
We then added well worded pages on my website, and it works really well. Many of our bookings was down to the fact we do everything in-house, and don't sub any of the work out.
A well worded website will work wonders. I personally have no issue with having other avenues of cleaning on my site. I ended up selling my domestic round a few years ago, as most of my work is carpets and end of tenancy cleaning (which incorporates windows, oven cleaning etc) .
It's personal choice, but it has not stopped me just getting domestic or commercial carpet cleaning.