Wow, some people are defensive... I said that I think it's selfish to "plan" for more children than you can provide with personal, private spaces in acknowledgement of the fact that things don't always go to plan. However, when you can avoid that (by limiting reproduction, or saving to move or modify your property), I think it's selfish not to try your best to do so. Why is it that some people seem to think that their offspring's claim to personal space and financial support with their studies is less important than their own want for more children than they can comfortably provide for?
And there's a huge difference between the late 20th century and the 21st, anybody who says otherwise is either way out of touch or living in denial! The labour market faces challenges now which didn't exist even five years ago, which I won't go into here, so it will be much harder for young people to get and keep jobs which pay a living wage. The disparity between house prices, earnings and availability of credit is far bigger now than it was in the 1990s and early noughties, and it's going to stay that way for the foreseeable future. The rental market is already inflating in the bits of the country where work is available, in order to reflect that. The vast majority of young people now have no hope of buying a house in their early 20s without parental support, and lots of them won't be able to rent either (unless their parents are prepared to sign as guarantors on the contract while their offspring struggle to find employment or work on zero hour contracts with no security). The result is that (barring unexpected personal crises) parents need to plan for their children to be able to come home for long periods in their early adulthood, and I don't think that sharing a room is sustainable when you're talking about people in their late teens and early twenties.
Obviously not everybody will be able to manage this (due to unexpected pregnancy, illness, redundancy, divorce etc) but I think that it is a fundamental responsibility of parents to try, in so far as their circumstances allow them to. I think it's great that the OP is thinking about what is reasonably achievable while her children are still young, rather than waiting to see whether her current situation becomes unsustainable.
I'm sorry to have touched a nerve for a few people and I realise there will be some who simply do disagree - all I can say is that I'm extremely grateful that my parents did plan their family around their financial means, as it has allowed me to grow up with the space I needed to study (and not resent my sibling), leave university with no debt, get an excellent degree in a good subject, and learn another skill to professional standard so I always have a fall-back income stream if I'm out of work. It also means that my husband and I have somewhere to go when our postgrad courses finish in September, while we sort out our next steps in life. My parents really struggled and suffered at times in order to achieve this for my sibling and me, but they wouldn't think of parenting in any other way - and, having seen the extent of the benefits, neither could I.