The difference is structure — LLC, insurance, bank account, records, pricing system.
A side hustle is cash work with no structure — it pays for the weekend but builds nothing. A business has five components: a legal entity (LLC), insurance (general liability at minimum), a separate bank account, records (scope, price, receipts for every job), and a pricing system based on real costs.
Structure built while employed costs almost nothing in risk. Structure built after you go full-time costs everything. The employed tradesperson with a properly structured side business has already crossed half the GAP.