Properly executed cladding should not cause any issues, but I have personally seen first hand that badly installed cladding literally destroyed a building structurally by creating a great environment for fungus to spread. Before making your decision you might want to go over this official guide on cladding: http://www.fema.gov/media-library-data/20130726-1707-25045-0913/chapter9.pdf It gives a lot of information on how cladding works, behaviour of water on cladding etc. I would personally never use cladding, but I have seen cladded buildings doing absolutely fine for 25+ years. Saying that they were in mild climates with no coastal winds or airborne salt. I'm not an expert on cladding or coastal properties. Insulation on an old house is always a good idea, as it really helps with cold spots and can greatly reduce fuel bills. With smooth render, you can also add additives to it to help inhibit salt, which would increase its lifespan in your area.

Is the black cladding wood or timber effect? Black is pretty neutral so I would decide what shade of brick you like first. Would it be possible to visit a brick manufacturer or does your architect have brick slip samples? There are many different colour variations from reddish, more yellow to grey. You need to see samples in the flesh to decide. Its an expensive mistake to make if you get it wrong. Look at as many examples of brick barn conversions as you can first to get an idea. I prefer stone to brick, especially blue lias. I only really like historic brick on period houses.

Has the work already been carried out? Certificate of lawfulness suggests so. If this is a planning application then the planners should be able to advise on what might be acceptable changes. Is there an appeals process? Might be worth engaging a planning consultant who can advise. Planning departments can vary considerably in how they apply the rules, some areas are fairly flexible, others less so. If you’re in a strict area a landing consultant who knows how that area operates would be worth considering. Also, changing the roof may not be compliant if the new roof is vastly different to the original.

It’s a bit hard to see from the picture what the colours are. I think you will have to have some form of cladding so could it be in real wood that will age to a nice silver (rather than plasticky finish you have now). I think that would look more classy along with a crisp white finish on the render. The arched front door is also very dating, if you will be keeping it in that location then I would consider replacing the arch for a regular door with side lights. The windows could go grey or green if you are updating. Lots to think about!