I bought a couple of oil radiators from aldi, makes a big difference in mid winter, well worth the cost.

Hi Stephen, I generally work on my guitar builds in the winter, in the house. I have a spare room that is converted into a small indoor workshop. However, all the major cutting, such as bandsaw, routing etc, I obviously do in my out-buildings as they are very messy and noisy jobs. As soon as those jobs are done, I bring the guitar back into the house to continue working on it. Not everyone has or wants a permanent indoor area for working on their guitar projects but there is nothing to stop you setting up a temporary work area on your kitchen table or back bedroom desk. A lot of the jobs can be done indoors, it’s only the big messy cuts that need to be performed in your shed or even outdoors on a nice day. 
If you are going to take this advice and work on your guitar project mainly indoors, then keep your wood indoors. Once you have measured and marked everything out, take it outside and make the cuts, then bring it back indoors. 
Alternatively, if you want to do it all outside, keep the wood outside in your workshop/shed. Your humidity levels sound quite stable but obviously the temperature is lower. The wood will be fine but you may be cold. Maybe a bad idea would be to fluctuate the heat in your shed, the wood may not like that and you could have some problems. I think wood likes stable conditions, not the constant changing of heat and humidity. If you do it fast enough by going from the warmth of the house, into a cold workshop and back indoors again (within an hour or two), that will be fine. That way you get to stay as warm as you can and your guitar build will be fine. 
It’s worked for me for years and I’ll continue to do it this way until I get some form of permanent heating in my out-buildings. 
Another approach is to plan ahead. If, like me, you start to build many guitars and you can’t stop (addicted), then get all of your guitar build projects cut out, routed and sanded during the Spring, Summer and Autumn. You can then get as many of them painted etc during that time and the ones you don’t get finished, you can continues to work on indoors during the winter. You could end up with six necks that need fretting, bodies that need the electrics soldered and much more. 
I don’t know how much of this will be helpful to you but this is generally what I do. 

Cheers.

Boo.

I keep my wood etc in the house, I take whatever I'm working on out to the shed, do what's what then it comes back in the house. I glue up in the shed then into the house it goes. Shed door is generally always open while I'm out there so it's more or less the same temp as outside without the wind chill. My shed has zero heating or dehumidifying, starting can be tough but I find I soon warm up once I get going. I'll carry on right through winter because I can't sit still! 🤣 

Thanks everyone for your input. its appreciated. Building in the house is not an option. Heating the shed to a suitable temperature full time is possible - but i might be bankrupt by the end of winter. At the moment I really  just keep the shed heated to a place above the dew point. Storage in the house could be good but its a 100 metre trek from the house to the shed. (no I do not live in a mansion, the garden is just very long).

I came across a site called "biggerjugs" (insert your own joke here) - its about brewing where creating a stable environment for fermentation demanded the building of temperature controlled boxes. I think i will have to construct something similar for storage of work in progress and raise the temperature in my workspace in a run up to a session in the shed. Of course by the time I organise that spring may have arrived 🤣 

Looks like i will be hiking this winter!