By looking at the layout of the current plan you can definitely achieve a lot with it, at the moment the layout is not great as you have an entrance hallway and than another hallway to enter towards sun lounge and workshop entry is from garage or garden. Kitchen itself is not very practical it does not have dining space big enough or even adding a kitchen island just would not work. Suggest you have the rear extension to square of the building towards the left and where the garage is to try and get the permission to bring the garage wall towards the front of the building of course convert the garage to play area or gym or maybe cinema. New Extension should have 6 panel bifold low frame doors for indoors/outdoor look and two large skylights. In the new rear extension towards the left, add L shape kitchen with large 5 to 7 seater island, open up the walls from sun lounge and workshop make all open plan for new kitchen, dinner and lounge, Relocate stairs to the left where kitchen is now and have double glass doors where stairs is now so the view from entry is right through to rear of the house and you can see the garden. You could have pantry, another kitchen for all the dishwasher, washing machine or even cooking so than the main kitchen is kept for socialising perhaps a wine bar or wine spiral cellar(amazing feature with wow factor) ? Just some of the ideas..

Its hard to give a full breakdown of options without studying the full plans, your requirements and budgets. This is something you need to do with your chosen Architect. However, there are many options for adding windows and ways of making light bounce around the house, but if your architect has said you can't add more glazing you should ask why. I would suggest it is because it will start to impact the thermal efficiency of your house. North facing windows get no solar gain so by adding too much you could end up with a house that is difficult to keep warm especially if your house is older. You could upgarde to super energy effiecient glazing but of course that will come at a cost. Having looked at your existing plans it looks like your house has already had extensive work done. Unfortunately it doesn't appear like an architect was involved. A lot of the cost of your renovation will be in correcting previous mistakes and even then you will end up with columns in ever room unless you are willing to pay to have that corrected too. The cost could make the option of moving more viable? Did the previous owners also extend upstairs?

It will cost the same to put new water pipes to the extension as the existing bedroom to the right. The difference is the cost of digging drains and I suspect that if a new bathroom follows your plan and sits in the top left hand corner you will have to have a new drain across the back of the house. I think you need to establish where the drains are so that you are sure that your “practical and financial” argument for putting an extra bathroom in the extension is actually true. From experience I know there is a good chance they go down the right side of the house in which case you could save thousands. Your deeds might include a plan or you can look for outside clues such as inspection covers

I'm no expert but have a similar floor plan in my own house upstairs. I don't think you will fit wardrobes at the end of the bed in your new extra room with en-suite. A standard bed is about 2m long, so if your room is 3m wide there will be not be enough passing space on a standard 60cm wardrobe. I suggest this extension space upstairs needs to be reconsidered. Hopefully one of the designers on here can help with a new plan.