Start with permission you already have a right to ask for: every customer at invoice time ('want the seasonal maintenance reminders?'), every quote, every service call. A service business touching 30 customers a month builds a 350-person list in a year from workflow alone.
Then trade value on your website: a genuinely useful freebie (pricing guide, maintenance checklist, cost calculator) behind a simple form beats 'join our newsletter' by an order of magnitude. Make the magnet worth the email. A thin PDF teaches prospects your promises are thin too.
What not to do: buy lists (deliverability poison), scrape, or auto-add everyone who ever emailed you. A small list that opted in outsends a big one that didn't. Literally, because inbox providers score engagement.