we have a double door kitchen unit under the sink. The sink sits entirely in one half of the unit. We would like to change the open section with a set of drawers. Is it possible to do this or is it a new carcass? If a new carcass is needed, how easy is that to do when it’s a unit within a peninsula, with a quartz top attached?
Pictures?
Sounds like you need to swap carcass or get inventive
Is it possible to do this or is it a new carcass?
Very possible, but it's a custom/DIY job.You could pay someone to do it, but realisticaly it might work out more expensive.
How are you with screw drivers and jig-saws?
If you can still purchase drawer fronts that match the rest of the kitchen then I’m sure a decent chippy would be able to make some drawers and fit some runners to fit the existing unit.
Otherwise it would be a real ball ache to fit a new carcass into an existing kitchen with the sink and plumbing to work around.
You either need:
Drawer fronts,
Drawers (easy to buy)
Runners (easy to buy)
Or something like this
With either you need to do some DIY to create something to attach the Runners to in the centre of the carcass. They're not that hard if you are OK at woodwork but how hard depends on carcass construction and how neat you want
OK link no good, search for
KuKoo 4 x Kitchen Pull Out Soft Close Heavy Duty Baskets, 600mm Wide Cabinet, Slide Out Wire Storage Drawers, Pull-Out Organiser Metal
If the back of the cabinet is solid then the runners can hang from the central support upright at the front, and these brackets screwed to the rear.
Ok thanks all, I’ve got some spare panels that I can probably use to create a new upright / wall within the carcass, then will have a bash at making the drawers, but getting matching fronts may be tricky
Unless you really fancy making, buying pre made drawers and a matching runner kit is way easier
Very possible.
The easiest way really is to make a box of drawers that you then sit into the existing cabinet. It may even be that you can buy something that'll work with minimum mods from somewhere like IKEA.
The thing to remember is that when kitchen doors are open, they tend to sit inside of the outline of the carcass. This means that any drawers need to be a bit narrower than the carcass in order to clear the door and hinges at the sides when opened.
When I build drawers into cabinets with kitchen type hinges, I will usually use a filler trim to the hinge side of around 30mm which is usually plenty to clear.
Also remember that the drawer box needs to be shallower to allow for any handles on the drawers and that.

...but getting matching fronts may be tricky
As NeilNevill notes, drawer fronts aren't really necessary if you can't find them. Just hide the drawers behind the existing door. For example: https://www.drawerboxes.co.uk/internal-blum-metabox-3-kitchen-drawer-pack-1-x-shallow-2-x-deep-c2x30835955
Drawers and runners can be found here: Furniture fittings, architectural hardware, electronic locking systems - Häfele (hafele.co.uk)
and will be better quality than any of the big name stores.
The trickyest thing is finding matching drawer fronts, but if they're behind the existing door it ain't a problem.
The units may well be drilled to accept drawers already. The drawers may need to be set back a bit to clear the hinges, but that really ain't a problem...
I fitted some internal drawers in the kitchen in our last house. In the previous IKEA system you could buy an aluminium drawer front for the standard drawer box to make it into a internal drawer. It's not always obvious how to order the right parts from the website but going and having a look in a kitchen shop may be worthwhile.
B&Q also sell internal drawers like those Blum ones up there.
You also need to swap the door hinges for 160 degree ones as a standard hinge doesn't move the door as far sideways so the door stops the drawer opening.
IKEA hinges and fittings used to be rebadged Blum. Not sure with their newer systems.
One issue I remember is getting hinge positions and drawer depth so that runners didn't need to mount on the same holes as the hinges took a bit of planning. Worked well though.
Buy a small cheap bedside cabinet and use that.