The second important aspect to equity investment is time; the time to make sure your money has a chance to grow in the first place and then the ability to defer withdrawing your money if something happens suddenly in the market that causes prices to fall. 

Most experts suggest that an investment in the stock market should not be undertaken unless you have at least five years to go before you will need the money. Assuming that in any one year, shares will outperform your building society account is wrong. However, as your timeline extends, the possibility that it will outperform increases. 

Of course, if we could all predict which years would be the best in the market and which would be the worst, there would be no issue with time. But we all know that is impossible.