Sanity has raised a big point, regional prices. If you take an average 3 bed semi as your benchmark, whats the going rate in your area?
Here in the London suburbs its about £15 but there are still window cleaners charging £8-10. My friend John had a window cleaner for 14 years who charged him £4 and never put the price up in all that time. He probably still is charging £4 but John moved house last year.
Sanity mentions £7.50 for Leicester, that seems a bit low to me but lets go with it for a while. If you price a house that is bigger you estimate how much bigger than a standard semi it is and scale your price accordingly. So a "house and a half" would be £11.25 and a "double house" would be £15. This effectively is called banded prices.
As you get more experienced you start to look for other things. When I was trad a big problem for me was cars in the front garden. So many house round here have paved their front gardens and park several cars right where you need to put your ladders.
You can waste loads of time while they move the cars back and even then trying to manoever ladders around expensive cars is stressful.
Access is another problem, recently I took on a new customer, access to the back was through the garage so no problem. I won't do "carry throughs" anymore.
The garage was a bit cluttered but she said she would get her husband to clear it. First time I went the garage was jammed solid and it took her ages to clear a space through for me. "Sorry, I did ask him to clear this."
Went again this week, took ages to answer the door, "Sorry, I was upstairs changing a nappy"
The garage was worse and it took even longer than last time to clear. Then the key for the garage door at the back was missing and she had to phone her husband to ask where it was, then she had to go and find it.
The time lost in all of this was about 20 minutes. Time is money in this game. I should have seen this one coming but I didn't. If its going to be like this every time I go there the job is underpriced.