What type of wood treatment does my garden office / summer house need?
Generally you want to treat exterior wood like this with a water-proofing treatment at least once a year. It helps to provide UV resistance and keep water out.
Water is your big enemy here. It looks like the wood was installed with some lacquer for waterproofing (hard to tell from the photo), but it's worn away. You've got a couple of options
Paint it
Paint will waterproof and make it look nice and uniform again. It sounds like you want the wood look, but as far as options go, this one is the easiest and had the fewest questions on maintenance. I mention it for completeness.
Stain and waterproof
I'm assuming you just want to work on the greyed spots. This is trickier because matching stain is difficult. Just like painting, your best bet is to stain the whole thing. The next best option is to find a stain that's close enough. Because you have a natural break in the soffit and wall, a close enough color might do the trick, and people will assume the wall just looks different due to lighting.
Then you'll want to waterproof it. This isn't a &quot;one-and-done&quot; either. Your wood will grey over time. The more often you waterproof it, the longer it will stay the same color. Waterproofing also helps prevent the boards from warping and splitting over time (water is your enemy there).