Exterior crack

weeg

As others have said, without seeing the whole elevation/ house it's hard to judge. 
That said, a single, isolated crack would tend to imply there is an issue with the lintel, rather than something more widespread. It's clearly a long standing issue, judging by the mortar. It's not something which would particularly bother me if that's the only crack there is. (Inserts disclaimer that I do inspections as part of my job, but in no way should you take me word for it)

Missy79

@Doozergirl, @FreeBear, @weeg
Apologies in advance for the length of post, I've added a number of new pics.

Thanks for the responses so far. As lockdown has now eased I have, in the first instance, got a level 3 RICS survey and gone back to look at the exterior again. Appointment Monday to look at the interior again.

So, the survey suggested that the main house was sound but did note additional cracking which we hadn't seen at viewing. Again to do with the extension. We have no idea when the extension was added, there's no planning docs, no building regs docs and the property was part exchanged so we can't ask the previous occupier anything. The copy of the title we have seems to show prev occupier from 1999, planning docs on council portal from their work but can't find anything pre- 2007 or so for this property. The surveyor didn't fill in section C giving an overall opinion of the property so I'm going to try and get hold of him by phone to ask further questions. All aspects of the property examined were put in the level 2 category and all described as "satisfactory".

Here is an image of the extension wall from the rear, the original crack runs down the centre.

There is the stepped crack to the left of the window: 

and a crack below that window:

There's a more minor crack/stepped crack above the right hand side of the patio doors:


The surveyor didn't comment on it but we also noted some cracking where the extension joins the main house:


The bricks do seem prone to cracking, there are a number with fine cracks that don't extend to the mortar joints and quite a few spalled bricks particularly on the left side elevation of the extension. It's unclear whether the obviously different breaks have already been replaced due to spalling, certainly the mortar joints don't appear different to the surrounding joints. 



The survey said little else regarding the cracks, section F3 on the interior said: "There is evidence of movement to some of the internal wall joints / abutments to the rear bedroom over the rear extension. The cause(s) of the cracking may be historic but this is not certain. You are advised to monitor the cracks for a period of nine to twelve moths, and to fill and decorate if they do not
deteriorate."
But section section J2 said:   "We saw no evidence of movement in this property."  

I appreciate that further investigation may still be warranted but opinions would be greatly welcomed. Not least because it might help me to work out what questions to ask the surveyor.