I personally think that if you are business minded and put enough effort into it, then yes £30K pre-tax is easily possible. Probably not in your first year of trading though,.. but it's a target you should have no problem reaching in your 2nd year even if you are just a one man band.
Beware though of the costs of setting up and running a business. What money is coming in may seem a lot at the start,.. but it really isn't all yours!
Tax and eventually VAT with take a big chunk, as will clothing, equipment, consumables, vehicle costs and fuel costs.
35% of everything I earn goes straight to the Government through Tax or Vat etc.
I spend on average €110 per week on various work related items (new poles, resin, workwear, scrims etc) and then there's diesel on top of that too.
My first year in business I was badly organised and not too bothered as I'd just made a profit selling a house. Still I turned over €28k
That year I was surprised when the accountant told me €20K went on running costs (mostly vehicle, fuel and WFP setup)!!
€8k profit in a year sobered me up a bit,.. especially as my savings were almost gone!
Starting out is tougher than it looks at first.
My advice is to get a separate business account straight away. Pay yourself a "wage" out of this account AFTER all other bills are paid. Keep an eye on your goal, and regularly work out how close you are to achieving it, and also work out exactly what your costs are for each week. DON'T spend more than you need to,.. A thermopure unit and brand new van can wait,... for now spend as little as possible and you'll still be able to do the same job.
A huge factor that limits your earning potential is your location. If there are lots of window cleaners locally, or if prices are just generally low, then you will have to be in some way competitive to win work. To get the best possible prices, you have to offer the best possible service. Be regular and reliable, dress smart and adopt your most professional manner,... it all helps!