Just register and pay your dues like everyone else has to. Neighbours could be a problem but so could other window cleaners if his? tax evasion helps him to work more cheaply than them. Assuming that the normal employment more than covers the tax allowance, all window cleaning money earned is subject to tax and NI. However, any expenditure incurred may be offset e.g. a portion of the car running costs (and possibly a portion of its value at the time of starting the business if it may be treated as a capital asset), the water and containers, any purifying equipment, stationery, a portion of the computer/printer if keeping records digitally, scrims/microfibres. It's also possible for tax relief on cleaning work clothes and cloths. However, without registering, you/he won't be able to offset any of the above. I suggest you find a cheap accountant (whose fee is also tax deductible).