Is it normal that only half my week is billable and the rest is admin Admin & Taxes

A few things that moved my number by maybe four hours a week: batch invoicing to one afternoon a month, a proposal template with three fixed packages so writing one takes 20 minutes not two hours, and refusing to redesign my own site more than once every two years. That last one is the biggest and I say it as someone who has broken it twice.

19 out of 40 at eight months is completely normal and honestly not bad. Six years in I average 24-26 in a good month and I've stopped trying to push it higher, because the non-billable half is what generates the billable half. The number that matters isn't utilisation, it's whether your rate times your realistic billable hours covers what you need with margin. If it doesn't, raising the rate is a much easier lever than finding six more hours in a week.

The people billing 30+ consistently are usually on long retainers with one or two clients, so their sales and proposal time is near zero. That's a real strategy with a real downside, which is that losing one client removes half your income overnight.

Worth separating admin you must do from admin you chose. Bookkeeping is a couple of hours a month if you keep it current, or a full weekend in January if you don't. An accountant for a few hundred a year buys back more time than most productivity changes, and gets the tax bits right.