What colour will the metal cladding be? White plantations shutters would just look wrong, with the other finishes on facade.
If the cladding is dark. Then dark roller blinds, or black shutters.
Shutters do not protrude into space, they are usually installed flush with the window.
What colours are you doing inside the bathroom. Would black rollers/shutters look good?

Thanks Dreamer - good point about the cladding. It will be black (or at least a dark dark grey). The rest of the house will remain as red brick, with the accents in cream and a dark grey to match the metal.

The black shutters look great, but we are hoping to keep it light inside the bathroom. The other thing is that we wouldn't want black in the WIR as we are doing a colour on the cabinetry.

I'm glad shutters do usually sit flush - I must have seen some bad examples!

Do you know how rollers hold up against bathroom humidity?

I have had no experience with roller blinds in humid areas, sorry.
But with roller blinds, they are either up or down. Shutters can be moved to allow light into the room, without outside being able to see in. Roller blinds, will not be private, or I suppose you could have them open enough to only show up to your knees.....
I would suggest to, contact a window treatment company, and get to know what colours, or best product for your dilemma.

what about ribbed glass instead of frosting? It was quite fashionable in times gone past and is making a comeback with all the black steel windows around
https://images.app.goo.gl/U2XXmYCXDvPGXinW6

https://www.blindsonline.com.au/zebra-roller-blinds

These blinds are so versatile. They give 3 possibilities, total privacy, looking like Venetians, or rolled up. I have throughout the house. Love them.

Those zebra blinds look great. I haven’t seem them before. I have blinds in our holiday house. No problems as long as you let then dry to discourage mould. But that goes with any bathroom.
You will have to either put a sheer covering or some short of obscure glass, doesn’t have to be frosted. There are all sorts of patterns.

Look at something like Veneta blinds they can be set to say on cover part of the window leaving the top clear for head view but not exposure:)

This fluted glass is what I meant

https://www.homestolove.com.au/fluted-glass-19451

Looking at the outside, why are the windows in the extension different from the other windows? Is it an aesthetic choice or a pragmatic one? Since the windows are different, why not make them a statement? Do something creative with the glass . There are all kinds of coloured and textured glass. Go talk to your local glassworks. Or investigate stained glass options. You can even paint glass with special paints for a stained glass look. You might be able to find a local glass artist who could design you something stunning. Look on it as an opportunity, not a problem to be fixed. And compare prices. You might find an option you can afford that turns the windows into a wonderful feature.

However, if you really want a window covering, I would recommend shutters. We have recently put in plantation shutters in our back sun room. We love them. They changed the whole look of the room for the good, provide absolute control of light and privacy, and are much easier to clean than venetians. You can get them in any colour you want if you pick the right company and they are very cost effective in small areas, but be aware that they measure in place for accurate sizing and the actual shutters are custom made in China. Ours took over 3 months to arrive, due to Covid. Then we had to wait for the installer another week or so. But they were definitely worth the wait. They are hinged so you can open them out to easily clean the window glass, and the shutters themselves just wipe clean.

We’ve always had internal shutters in the bathrooms. Both mid coloured timber coloured ( not that dreary red brown of old) and white at various locations. For the 96% of the time that no ‘private business’ is underway we keep them open, and a quick flick when needed and they are entirely closed. In winter I swing them away off the windows and enjoy the low sun. All depends on your outlook from the room. And aesthetics. But for useability they are great. So maybe mid coloured timber might work?

Bathroom windows at the front are not very good, you’ll always have to keep blind/shutter closed even during the day especially as you have toilet in there otherwise you run the risk of someone seeing you on the loo.

Thanks for everyone's thoughts and suggestions. We're pretty convinced shutters are our first choice (pending budget allowance) and we'll get the ones that are split half way (so that the bottom half of the window can be closed, while the top half of the shutters are open/slanted). We've been stalking houses around the neighbourhood and have seen this in action and seems to work well.

Here is a concept that we use frequently when a toilet or bathroom is facing the entry.

Total Renovation · More Info

Those windows are wrong you shouldn't add windows and then have to work out how not to be seen day or night, It would have been better to use high wide narrow widows in both the ensuite and WIR

siriuskey

These windows the same colour as cladding won't need any window treatment you are getting the same modern window profile turned sideways