Impossible to be certain from here without context. Does it carry thru to the interior? What is the wall constructed of?

Hey thanks for the reply. I can't see what's on the inside as it's wallpapered over and currently a pretty full pantry. But, I do know it's brick built and the home turned 100 years old this year, I have the deeds.

You are going to need someone on site then to make a determination. Could go either way and no realy way to guess without seeing in person. And realistically to properly fix that would require inter and exterior work to be proper. Assuming a fix is required.

I see from the sockets you're in the UK or Ireland. The cracks are in a solid brick wall. It's not just the mortar. There is no render and no siding. It's just painted brick. The bricks themselves are breaking. This isn't a plastering job.

It appears you're trying to build an extension onto an already dodgy rear wall. I'd stop that project immediately and get a proper reputable builder and or surveyor in. It looks as if you have subsidence or building movement. Btw the extension looks badly built too. The rafters shouldn't be cutting across the frame of the sliding doors.

Are you trying to accomplish a cosmetic repair? What is the goal here please?

A crack under a small window usually follows the opening. The sill is a stress point. If the wall moved, plaster and chicken wire will crack in the same place. Rebar set into a plaster coat does not tie a foundation or a lintel back together. It is a stiffener in a finish. If the crack is still moving, the bar does not stop it.

Before anyone plasters, find out what the wall is and whether the crack is active. Masonry, stucco over lath, and a rendered block wall are different repairs. A hairline in the finish, same width top to bottom, with no offset and no soft material behind it, is often a finish crack. A crack that is wider at one end, steps through the joints, or has one side proud of the other is movement. Mark the ends and the width, and look again after a wet season and a dry one. An active crack gets diagnosed. A stable finish crack gets cut out, the base repaired, and a proper mesh in the patch, not a sheet of chicken wire over the top.

Also look at what the window is doing. A sill with no drip, failed caulk, or a head that dumps water into the wall will open a crack under the opening even when the foundation is fine. Fix the water before the plaster. If the crack lines up with a corner, a downspout, or a low spot in the grade, that is the place to start, not the rebar.

The Handyman Calgary does this kind of exterior crack and flashing repair in Calgary and nearby neighbourhoods. 587-324-7566. https://www.thehandymancalgary.ca/