For anyone who interested - this was the idea I threw in... Using the space in a slightly different way !! A. Multipurpose kids play room / guest room with bed option either as Murphy bed or sofa bed - with plenty of storage for toys etc (if this is currently a kitchen - with higher level window - suggest replace window with lower one to match the opposite window in B ) also - if kids not too small - could keep a vanity sink in here for guests - if this was the kitchen. (If you need bigger play room - suggest maybe knock into garage too - but keeping front third of garage as garage still for garage type stuff. Subject to structural /fire regs etc of course ) B. One child's room with built in wardrobe C. Second child's room with built in wardrobe D. Hallway entrance with cupboard storage (pink ) on both sides - suggest new double glass doors opening into new dining room and extension at end of hall. E. Family bathroom . Pink cupboard opposite door for towels / bed linen /loo rolls and cleaning products etc . (Smaller pink closet for tanks if needed ?!) F. Master ensuite shower room . ( if no window possible - swap loo and basin location as neater to see basin first, not loo - but good to have natural light for shaving etc near window ! ) G. Master bedroom - with new window and glass door way through to garden access ( as fire escape - but also good for security as you can see anyone trying to access back door ) with his and hers walk in closets . TV on bottom wall. (Will need structural engineer to approve spans etc ) H. Formal dining space - or could be used as secondary snug if formal dining not required . Add side window. Consider making existing opening wider, so view into kitchen possible from front door. ( if structural engineers allow ) I. Glass roofed open porch area - ideal for outside storage /extra drying space / log store etc etc . Together with combined mud room / boot room / laundry room - access too and from outside and access point into kitchen. Could also be used as extra pantry storage too. J. Kitchen area - suggest island prep and pretty much everything area with tall cabinets flanking USA style FF and at end of hob run for max storage - DW on end of island nearest laundry room to be least invasive to kitchen flow. L shaper bar gives views of garden and is better for conversation than one long run. K. Lounge area . Can be two sofas - or one with pair of swivel chairs on garden side - remember to leave plenty of room to walk round - you don't want it feeling cramped - anchor sofas on large rug to help keep them in place and to provide comfy floor seating too. Could have extra (toy) storage all down the bottom, TV wall if you want ?! So that's my idea - I hope you like it ! It provides all you asked for I think - keeps the bedrooms together - I can see the children (if small) spending more time playing in the extension, rather than the multipurpose room to be honest - but if older they will relish the space away from direct view of parents. So they will grow into it ! If you need to grow the house again in a few years then maybe stairs could start off in larger built in closet to minimise disruption .

This comment is a bit late, but we bought a house last year that was extended in 1992 over not one, but two, manhole covers (kitchen and cloakroom). Both had been tiled over but with sealed hatches that could be lifted in case of emergency. In 25 years this was never necessary but the idea of leaving them in tact was a little unpleasant. It's a public sewer that includes several neighbouring houses. We just undertook an extensive house refurb and moved the manhole access points outdoors and permanently sealed the internal ones. It wasn't as complicated as we originally thought. What you want to do as a starting point is get a drain surveyor to come and inspect the routes and flow of waste. We used Power Rod (we are based in Hertfordshire) and they came with their camera on a hose reel and mapped out our plumbing routes all the way to the mains in every direction. Then your builder will be able to take it from there, or you can hire a specialist grounds company to sort it out. If it's any help, in your head you may have this idea that raw sewage is just gushing under your floorboards but in reality, when I looked down the hatches, it's really just a matter of a few pipes several meters down. It was surprisingly more sanitary than I expected and I found that reassuring.

I prefer the second layout it seems to flow more easily. Any reason why the external master bedroom wall doesn’t run along to meet the new sitting room wall? In terms of building it would be easier, removing the need for awkward angles on the roof which would also help reduce costs. I would also have bifold doors onto the rear garden rather than patio doors but that’s just my personal opinion taste.

That is very nice I must say, nice to see some different from the norm grey rectangular and wooden clad rectangle.