In most writing, sentences should be planned and structured into paragraphs. Paragraphs should consist of a group of sentences that address the same topic or sub-topic. While it can still be grammatically correct to have paragraphs are that lengthy (e.g. half a page or more), it should be remembered that readability is also important, and this is especially so for those reading off a screen.
Ideally, you will plan out where paragraphs start and stop before you start writing although, for whatever reason, you might find yourself just starting to write with the words just flowing. Regardless, for most types of writing, you should start a new paragraph for each point you are making or for each issue issue you are addressing. It is difficult to develop a good, well-paragraphed structure without clear thinking, so as already mentioned, it is good to plan ahead.
There are no rules regarding the length of a paragraph so, even though a single issue is being addressed, it may be that that issue should be further divided into sub-issues, each with its own paragraph, to avoid lengthy, hard-to-read passages of text.
It is good and common practice to start a piece of writing with an introductory paragraph and to finish with a concluding one. Related to this, it is often easier to write a good introductory paragraph as your last piece of work - once you know what you are introducing!
Paragraphs are used to group sentences based on their topic. This does not mean that they should be unconnected, indeed a well-written transition helps the story to flow. For example,
Money was tight for most families for that whole decade. Holidays, if they were taken at all, were enjoyed close to home and there were no trips to exotic places.
There had been years and years of stagnation in house prices. Even with interest rates being much higher than they are nowadays, houses were more affordable. This meant even poorer families were able to buy their own homes.
Rewriting the 2nd paragraph as shown below, creates a better transition around the concept of poorer families.
Money was tight for most families for that whole decade. Holidays, if they were taken at all, were enjoyed close to home and there were no trips to exotic places.
Even though the average family was poorer than today’s, they were more able to buy their own home. This was because, even with interest rates being much higher than they are nowadays, there had been years and years of stagnation in prices and houses were more affordable.