As a guide - the bigger the room - the bigger the furniture needs to be ! I have, for example , a pair of 8' ( four seater ) sofas in my lounge and the day I ordered them, three times they asked/checked I really had room for two of them, as they were the biggest sofas in the shop !! My lounge is about 6mx5m with 3m high vaulted ceiling - and the sofas actually look the right size, anything 'standard' would look too small in there !!!

One suggestion I'd make - shelves above a bed are very hard to reach, without standing on the bed. There are many beds though that fold up into a wall cabinet, so take up the thickness of a mattress/frame when folded up. It also makes for a tidier office when it's not a guest room. You may have to hunt for a fold-out that hinges on the long side - most are hinged on the short side (i.e. hinge under the pillow). If you can't find the right unit you might be able to swap with the shelves behind the door, or on the opposite far wall. You'd then have to rejig the other shelves you want above the bed, or move the bed unit the depth of the shelves out from the wall, or rotate it 90 degrees to parallel the wall to the credenza. I'm trying to work out exactly the same problem in my study as we speak :-) albeit with a different floorplan. Good luck!

Just looking at your plans again, any chance of doing a pop up roof both sides? Do you have to reinforce all the walls in your downstairs area - starting from ground level? I was just thinking in for a sheep as a lamb, if you're spending that much money can you possibly spend a little more and get maximum use from upstairs. It's so worth it, especially if you end up with a view of something/anything. Something else to bare in mind in insulation between upstairs and down which will stop noise travelling. I don't think you will need insulation in your upstairs ceiling as it tends to get very hot up there! We have a butanol roof on top which is supposed to last at least 20 years. One side of our upstairs are small verandahs which are lovely but rather a waste of space, but if you have any sort of view they would be incredible and increase the value of your property immensely - not to mention they are just lovely in the summer.

Get a cabinet maker to custom build a work station across the entire width of that blank wall. As it is on full show I would recommend incorporating mainly cupboards and drawers so you can hide stuff away. Consider using a coloured polyurethane and nice stone bench top and cabinetry hardware, as it will be the first thing guests see when entering your home. Contact me if you would like further design suggestions.