Here's how I would try to turn a £20,000 Stocks and Shares ISA into a small fortune

The 25th Anniversary of the ISA was celebrated this year, and many millionaires later, the Stocks and Shares ISA is still seen as one of the best ways for Britons to spend any extra money lying around. If such accounts are still with us in another 25 years, I dare say many others will have built up to millions of pounds and beyond.

The current limit is a maximum deposit of £20k per year. But the important detail is that there is no upper limit on the storage size of the ISA. Anyone who spends their full savings today can see that grow to a smaller or, in theory, at least, much larger amount.

My way

If I were hoping to follow the same path, starting with a base of £20k, I could aim to turn it into £400k. Such a large sum can make for a comfortable retirement. there will be thousands of pounds of monthly income in sales depending on the withdrawal rate. That sounds like a bit of luck to me. On the other hand, it means multiplying my first pole 20 times. That's not a bad thing even with the worst record the stock market has had down the years.

Let's say I wanted to hit my 50th birthday ISA figure and turn £20k into £400k over 25 years. However, that requires an 11.3% average return, above the stock's historical levels by a few percentage points. By choosing stocks wisely, the goal is not a complete dream. I would still call it a good investment even if I came up short.

The next question, of course, is which stocks to choose? My first port of call may be looking at non-core sectors. The UK banking sector is transparent. It has looked cheap as chips since the 2008 summit, and even cheaper since Brexit. The general outlook for most of the big Footsie banks in the country is still very positive. But banks might not be trading at such low rates if they weren't.

Strengths and weaknesses

HSBC (LSE: HSBA) is a stock I like and am considering buying. It trades cheaply by historical standards and its valuation matches it FTSE 100 competitors.

But the bank deals more with international businesses, especially in Hong Kong and China where there is more scope for growth. Banks, like many sectors, benefit from rising GDP. HSBC is well placed as almost half of its revenue comes from Asia.

The Chinese factor is a strength here, but it's worth remembering that it can also be a weakness. Geopolitical crises have erupted around the world. And if they get worse in the South China Sea then HSBC will be caught in the middle. A possible conflict with Taiwan could have serious consequences for the company.

The year will be 2049 when (and if) the ISA celebrates half a century. And while I'm not looking forward to how old I'll be by then, I'll probably be content with the money I've built up over the years with tax-free ISA Stocks and Shares investments.