From what I have read the 18mm MDF is OK with the blum hinges I have been looking at and I have found people on YouTube using 18mm MDF for carcasses so that's reassuring. I just need to have enough on them for the height so looks like it will be 4 hinges per door.
18mm is the modern equivalent of the old 3/4in (19.05mm) blockboard or laminboard which was used at one time for carcassing. It is pretty much a standard in the UK and is far easier to source than 15mm. another reason that German manufacturers use 15mm is that they can get more boards on a truck for export (weight and size)
I think it's less of an issue if you have the tools and space to do this in a workshop as you could make them with a very small gap and skim it off as required.
I seem to recall a couple of manufacturers who worked to a 3mm gap, but that is for pre-finished doors whereas yours are sized as raw MDF, so after they have been sealed, primed and painted I'd expect the sizes to "grow" 1mm or so bigger in real terms
...the intention is to get every hole and recess I can think of machined in by the cnc vendor, so it should just be like a giant piece of lego which I'm actually really looking forward to putting together.
Then remember to make allowance in your sizes for assembling flat on the floor and rotating up into position. Attic rooms are often difficult to get full sized furniture into - it is far easier to move individual panels upstairs and assemble it where it will live. Also consider how you are going to pull up the joints in final assembly - in a workshop you'd normally assemble on either a bench or trestles and clamp the item together using sash cramps whilst the glue goes off. Something the depth of a wardrobe (600 to 700mm deep) will require at least three cramps per end (these needn't be bought in - they can be home-made from 2 x 1 softwood). Finally have you considered the plinth? This really needs to be dead level, regardless of what the floor is like. I always favoured making up a softwood ladder frame by screwing 2 x 2 or 3 x 2 softwood together (a trick from bar fitting to make life easier), positioning that, levelling up using packers beneath the frame as required then angle-plating and screwing down to the floor:
Ideally I'd say your design needs the carcass to overhang the ladder frame a bit to allow comfortable standing hard by the cabinets without stubbing the toes. This rough frame is faced off by a rip of MDF which can be scribed to the floor (jigsaw, block plane) as you would scribe a kitchen plinth:
If you are lucky enough to have exactly plumb walls (checkable with a plumb bob or a laser) you will most likely be able to build straight onto the frame. I'd caution about making your carcasses tool big as the heavier they are the harder thet are to assemble and manhandle into position - better to build separates and join them together (clamp together on the front edge, align and screw beneath where the cruciform plates for the hinges will go to hide them - the ones at the rear of the carcasses will need to be countersunk, and filled with 2-pack filler afterwards)
One way to design carcasses is to set your top and bottom panels within the sides, thus:
This is commonly used on kitchen cabinets. In this form, without a back or some form of additional bracing the carcass is going to be very weak and can break the joints if lifted awkwardly, so some form of bracing or back, either temporary or permanent is advisable.
With out of plumb walls a scribe strip can be made up and goes between the end of the carcass and the wall. Obviously an allowance for this must be made when designing the carcass (here shown as 50mm). One way to accomplish this is to screw and glue a piece of 2 x 1 planed softwood down the edge of the carcass set back by the thickness of the scribe strip from the front face of the carcass (lt. yellow). In this case I have drawn the scribe strip as 12mm thick. Accurate positioning is essential and if you don't already have a decent combo square when doing this, you will need to get one (recommend a Bahco CS1560 or CS300). The scribe strip is cut and then scribed to match the wall (dividers/pencil compasses, jigsaw, blockplane) and is fixed to the front edge of the timber batten (glue and pins). The edge to be scribed is highlighted in red. You are aiming for a gap of 1mm or less all the wal down so that it will caulk in nicely. Note that the doors will need something like 2mm clearance between their back and the face of the cabinet - this is shown here and is what the hinges will give you more or less by default
This does mean there will be a set back from the front faces of the doors to the scribe strip. If you wish to avoid this a wider piece of 18mm MDF should be affixed to the side of the cabinet so that the edge of it protrudes 8mm from the face of the carcass (if using a 12mm thick scribing strip) to bring the face of the scribing strip flush to the doors. In this case I've illustrated the support strip as being 18mm thick x 50mm wide and I'd screw and glue that to the side of the cabinet before manouvering the carcass into position and doing the scribes. If you used softwood for the side of cabinet batten it would not paint out the same as the front of the scribe strip which sort of spoils the effect
To further disguise the joint in the MDF a shallow V-joint can be planed in as shown below. This is only 1.5 to 2mm wide and when you come to paint the carcasses you can take the doors off and "wipe" a thin bead of caulk down the length of the "V" with your finger. This has the effect of bith hiding the joint (which can show up as a crack in the paint over time) and of softening the V-cut to a shallow, rounded groove which is not very obtrusive, especially as it is half hidden by the door
About the only other bits of advice in terms of design that I can give at this juncture is that when designing these units bear in mind that a factory made unit is often 600mm deep, presumably because they are made from standard 1220mm wide panels, however I have always found that 600mm is a bit shallow for mens jackets. In fact 700 to 750mm may be preferable, but that may involve either excessive wastage or the purchase of larger sheets of material, such as "jumbos" (circa 3020 x 2000mm depending on source). Always go for MR-MDF over standard MDF, being denser it machines to a crisper edge. Lastly before painting break the arrisses (the sharp edges) just slightly with some P120 alox paper held in your hand - the slight radius will paint out better
