I've been told by our plumber friend we need to replace the nuts on the screwed in bolts with as per second photo. These secure the new terrace/pavilion frame to the wall. But strangely enough I remember he mentioned via my wife covering with a plastic cap. Or he meant a plastic nut .... can't quite remember.

I'm not keen to replace the nuts with as per photo 2 because I'm concerned it would invalidate the warranty we have with the building company. At this point in time the builders have been back to repair/correct some areas where rain water was getting through and running down the wall. So far no further problems. I'm concerned if the builders had to come back again they might point out the nuts had been replaced and it's caused the problem again ending up with myself having to pay them extra to rectify it.
My plumber friend has opted for No.2 for his pavillion but as mentioned I'm certain plastic caps was also mentioned some time ago.

Unless they are unsightly, I think I would just leave them alone for now. You could possibly paint them if they get too rusty.

Why did the plumber think the nuts needed replacing?

It looks like the studs have been cut to length with an angle grinder - this being the case they will rust, potentially giving rust stains to the wood. If this is the problem I would just paint the nuts and washers with an oil based paint. (Don't use a water based paint because rust stains will come through the paint)

Rust was an issue he raised. Yes, they were cut to length.
I thought of putting a bit of grease on them but I'm not sure if a good idea.
I better do some research into oil based paints.

As per Peter_in_Hungary I'll paint them with oil based paint.

I'd reccomend Hammerite brand paint which you should find in Praktika. I used it to repaint my metal driveway gates.

https://www.hammerite.hu/

IMO no point in buying a pot of paint just for a few nuts and washers. Any oil paint (of an acceptable colour) will do even oil based varnish. Whilst it is outside they will be protected from direct sun (I presume) and not subject to any wear so any oil based paint should last for years.

We still got the tin for the stain so will check out if it's oil based.

1. A small tin of hammerite black, or some waxy type anti corrosion protector (Like Waxoyl for cars) applied with a brush.

2. Spray it all over with WD40 and wipe away excess with a rag. Apply plastic blanking cap over the nut portion. (often used for pipes/drains)

For the plastic blanking caps I assume they would be available in a hardware store?

Yes.

Thank you.
I think we have one on one main road through our village.

The hardware store on the road going through the village didn't have the plastic caps so ordered via the Internet. Bought 2 x packets of 10. Only needed 13!
I couldn't reach one of them because my ladders are either too short or too long! Arrgghhh! I will have

to phone a friend to borrow his.

That looks neat. I have been trying to get the plastic collars / cones that you can fit onto pipe fittings that go into tiled walls. The chrome ones that come with the fittings never seem to fit properly so I end up leaving them off. These plastic ones clip together so can be fitted without disturbing the plumbing.

Our plumber gave us some plastic cones for the CH pipes. Can't remember where we put them.
The black caps are for covering the nuts for our verandah roof frame as per earlier photo.
Found these via Fruugo