I'm looking for some good sources (or just ideas) for multi-season plantings that also follow rotation by plant family.
We have six beds and are trying to maximize not on only space but length of season. Here in Seattle we can often grow spring and fall versions of the same crop. Sometimes the fall crop is about the same length other times it takes (or lasts longer).
Because of this I'm tying myself in knots trying to figure it out.
Unless you can find some advice tailored to your particular situation, go back to first principles.
Perennial crops (rhubarb, soft fruits, etc) are excluded from the rotation for the obvious reason, though if you replace old perennials with new plants, consider putting them in a different location.
There are five main vegetable groups in the rotation system: Brassicas, Legumes, Onion family, Potatoes and tomatoes, and Root vegetables except those in the first four groups - swedes and turnips are brassicas, for example.
Some vegetables don't need to be included in the rotation, and don't fit into the above groups in any case. You can grow them anywhere convenient, so long as you don't always grow them in the same place. Those include salad crops, cucurbits, sweet corn, and vegetables which are biologically in one of the five groups but in practice don't share the same pests and diseases - e.g. peppers and aubergines (potato family) and French and runner beans (legumes).
For the main rotation, divide your plot into equal sized areas, and allocate one or more of the five groups to each area. You don't need to divide the plot into five areas - for example you might have three equal sized areas, one for potatoes, one for brassicas, and one for the other three groups, depending how much space you need for what you want to grow.
Then, rotate the crops annually around the different areas, so that if you have three areas, you grow a particular crop (e.g. potatoes) in the same place once every three years.
Growing the same type of vegetables twice in one year (in spring and fall) is not too bad, so long as that crop is not grown in the same place again for 3 or more years, depending on the number of plots in your rotation. But obviously if your spring crop has some pest and disease problems (e.g. your spring brassicas get club root), you might want to change your plans rather than blindly planting another crop and hoping it will be OK! You might be able to swap over "rotating" crops and "non-rotating" crops in spring and fall to avoid growing the same type of crop twice in one place - e.g. brassicas and salad crops.
@alephzero has a nice introduction, but maybe too theoretical.
Manure, spade, and size of home vegetable gardens makes most of rotation irrelevant, but it is still a principle that it is not wrong, so if there are not other reasons, you should do it.
In addition, I think that diseases is also a problem. If you have root problems (or just not very moving insects), it could be better not to plant the same vegetable in the same spot or nearby. Garlic, onions, leek, and carrots could develop such problem (after years).
Now we plant more compact and with no strict zone, so rotation will be difficult. You may plant your winter vegetables e.g. between tomato plants, and on falls, when you remove tomato plants, you will have more space. In this case you will have less watering and less weed removal. This is much try-error and gardening-spotting (look around how other people are doing). Note: but in this case you should check which plant could be planted near which other plant.
