Wouldn't it be nice to sit back and watch $12,000 roll into your bank account each year without lifting a finger?

That's the dream passive income can offer — and it is more achievable than you might think if you plan smartly with ASX shares.

However, if you're starting from zero today, it is important to understand that going all-in on dividend shares right away isn't necessarily the best approach. Sure, high-yield dividend stocks sound tempting, but they're unlikely to get you to the portfolio size you'll need to generate $12,000 a year in a reasonable timeframe.

Focus on ASX growth shares

The smart move is arguably to focus on ASX growth shares first instead of dividend shares.

Companies that are growing their earnings quickly tend to see their share prices rise much faster than mature, slow-growth businesses. Over time, that compounding effect can turn a modest investment into a sizeable portfolio. Growth today means passive income tomorrow.

High-quality ASX growth shares such as Lovisa Holdings Ltd (ASX: LOV), Breville Group Ltd (ASX: BRG), and Temple & Webster Group Ltd (ASX: TPW) are forecast to deliver strong earnings growth over the next few years.

These companies aren't necessarily known for their dividends yet — but that's not the point right now. The priority is building capital.

What do you need?

The magic number is $240,000

Once you've grown your portfolio to $240,000, that's when the real passive income fun begins. You can start shifting your investments toward reliable dividend-paying shares. Targeting a portfolio dividend yield of around 5% will then generate that $12,000 a year in passive income.

Of course, you could tweak these numbers. A higher yield would require less capital, but typically comes with more risk. Whereas a lower average yield would mean needing more capital, but might offer greater safety and growth.

Which ASX dividend shares should you buy?

If you can invest $500 a month into ASX shares and generated a 10% per annum return (in line with historical returns but not guaranteed), it would take a touch over 16 years to build a $240,000 investment portfolio.

Unfortunately, it is impossible to say which ASX dividend shares you ought to buy for passive income in the 2040s. A lot will change between now and then.

But if I were building this income portfolio today, I might look at ASX shares such as Telstra Group Ltd (ASX: TLS), BHP Group Ltd (ASX: BHP), Accent Group Ltd (ASX: AX1), Macquarie Group Ltd (ASX: MQG), or even the Vanguard Australian Shares High Yield ETF (ASX: VHY).

So, in 16 years, it may pay to look for the equivalent of these ASX shares.

Foolish takeaway

Building a $12,000 per year passive income stream doesn't happen overnight, but with a disciplined focus on growth first, followed by income, it absolutely can be done.

As the great investors always remind us: time in the market beats timing the market. The earlier you start, the easier it gets.