E D not sure why there wasn't an option to personalise my user name it just gave it to me! Jonathan thank you so much for your detailed drawing it had never occurred to us to think about that layout which is why we posted on here Our house is built on a hill so our extension floor level is up around 1.5m and you have caught the shape down the side of our garage perfectly so would that all add to the cost of the build The layout does definitely give us something to think about as it wouldn't iinterfere with the light in our main living space and would be nice to have the option of the shower in there too

A floor plan might help

Think the kitchen should probably become an indoor WC and a utiity room (or perhaps you can squeeze WC under the stairs) and the open space becomes the kitchen/dining/seating - in which case I think you probably need to open up the space to the outdoors WC and include that in the open plan space. To get kitchen moved to the current sitting room will involve ripping up floors to get water/waste into that space, so it's not a small job. And no doubt you would uncover issues with the extension once you get going. Another option would be to open up the wall between kitchen and sitting room - maybe just knocking through an opening to allow you to insert an island where the current wall is (pillars at both ends of island and it wouldn't really be an island, more of a peninsula). I'd want WC under the stairs in that situation so worth checking headroom under stairs for that, and how to get plumbing there.

You may need an architect to help you. The design that has been sent to you in red maximises space without any consideration for the overall visual look of the house. It is difficult to plan a space without a full look at the surroundings including street aesthetic. If you want to limit the expenses on design, you could just ask for a design sketch with some massing details. This will set you up in a good place to work out the interior. I think it would be a mistake to just go for the biggest footprint without looking at how this sits on the site. It may well be the right solution for the large footprint, but you need to look at this with a balanced approach. If you have some photos of the main elevation and side, it would help in design. One thing that you should consider is how services are fed into your current home. Check drainage paths as often these are shared and may impact neighbours , which makes moving them to suit a big extension more difficult and costly. Electrical and gas supplies are equally a challenge,so try to get an understanding of these when enbarking on a project to ensure you dont waste huge chunks of cash on moving around services. They dont add value to your home and ultimately reduce the amount of nice to haves from a budget.