To plan or not to plan?

Fiction writers are firmly divided into two schools of thought; the planners and the non-planners. The planners believe in having every detail of a novel worked out before they start writing, sometimes to the point of knowing exactly what will happen in every scene.

The non-planners literally make it up as they go along. Some crime writers do not even know who the villain will be when they begin writing. I'm definitely a non-planner, although perhaps not so extreme as some.

When I start writing I always know who my main characters are, what the main conflict of the story will be,  and have at least some idea of how it will be resolved. But I don't know exactly how the story will unfold.

Two things, however, are essential before I can start writing.

I have to get the main characters' names right. I once changed the name of my central character because I thought the name I'd initially chosen was over-used. It took me a while to get used to her new name, and with it her character developed differently from how I had originally imagined her.

In historical fiction, I have to know exactly when the novel is set. Historical fiction centred on particular events, or a real person's life,  is of course tied to specific dates. But if that isn't the case, how does a historical fiction writer decide exactly when a story should take place?

When I began a previous piece of work, I knew the story had to be set in the late eighteenth century,  because that was when conditions existed for the story I wanted to tell. I decided to set it specifically in the autumn and winter of 1792-93,  just before Britain went to war with Revolutionary France. The impending war heightened some of the tensions and conflicts within the story, and the autumn weather, combined with the physical setting, could be used to establish the mood.

I'm just in the very early stages of thinking about another piece of work. The inspiration came from a passage in a book published in 1879, but the story could be set a few years either side of that.

Since it's to be a crime story, set in London, I don't want to go as late as 1888, as I don't want to have to deal with (or ignore) the Whitechapel murders. I also don't want to overlap with the Sherlock Holmes canon; the first story,  A Study in Scarlet, was set in the early 1880s and published in 1887.

I also have the impression that the 1870s are less used as a setting for fiction than the 1880s, so the earlier decade might be a good choice.

The year and even the month will be important as technology, and the
geography of London, were changing rapidly in the 1870s and 1880s. In 1874 my character could have seen the demolition of Northumberland House, the last of the great noblemen's palaces which had once lined the Strand, followed by the construction of Northumberland Avenue.

From 1878 she might have seen a telephone in operation in London. The Royal Albert Dock was opened in 1880, the Circle Line was completed in 1884; in the years preceding my character could have seen them being constructed.

Quite possibly, as I research the period, I'll discover something that was happening in London at this time which I can use in my plot, which will determine the precise date the action takes place.