I don't know why your decorator said that, unless he hoped he'd get the job of doing the continuous repainting!

If you want to render it then a through coat render is much better than a sand and cement render that you need to keep repainting after every little chip. A through coloured render will have the colour running all the way through it so if a little chip or dent happens you won't notice so much as you would in a painted render (depending on the colour you paint it)

There are lots of different render options. Some are better than others, some are even self-cleaning!

Often buildings are rendered only down to cill height on the ground floor because you're more likely to get things bashing into it lower down, which can cause more low-scale damage that then needs maintenance.

I'm not a fan of painted brickwork because it flakes off and can look terrible. It depends on what you paint it with though - some kind of lime whitewash might have a softer effect and be a bit more like a render finish so a bit of a compromise option. This would give a pale colour for the wall at ground floor level so the grey windows contrast with it, like the white windows above contrast with the brick.

Alternatively you could stain the brickwork, which is permanent. The bricks above are quite a dark multi-mix so you could pick a stain similar to the darkest shades in the original brickwork to tie in with those and work with the grey framed windows downstairs.

Either way you need to make it look intentional for it to work. Hope this helps!

Perhaps limewash and paint upstairs windows to match bottom windows

I’m no expert but I think technically there’s no problem having it rendered (I agree with I-architect about your builder’s apparent view) and I’m sure render can be painted.

But from an aesthetic viewpoint I feel you may well be right to be concerned.

I would lean towards painting the brickwork.

This would indeed bring the two parts together but still with a (gentle) distinction between the old and the new.

When properly done (I suspect it’s all in the preparation), painting brick should look good for many a year.

We have a home where it was done decades ago and for as long as we own it (around 7 years) it has not been necessary to repaint and is still looking good.