Completely step free access to the house and garden. You don't know how good that is until you're carrying heavy shopping/children/twist your ankle/the deliveryman/engineer remarks how nice it is.
An extra wide front door. Not just for somebody using a wheelchair/crutches - it's also brilliant for carrying lots of bags, bringing in appliances, buying a sofa and not worrying whether it's going to fit and it just feels nicer to be able to open a door wide and have tons of space.
A layout where somebody using a wheelchair or crutches/sticks can move around freely without getting caught on awkward corners or small steps.
Solid wood and stone/slate flooring. No MDF, polystyrene or plastic where it could be done with wood, wool, haybales or other natural materials.
Wide stairs with a strong banister running the entire distance - not just on one side or that needs pinch grip strength - you need a proper rail all the way on both sides. And good lighting in the stairwell.
Large, modern sash windows. I love the things, how they look, how they ventilate, but obviously with less maintenance involved, better security and not from uPVC - real wood.
Large tap turners. Not 'institutional' ones, but where you don't need a good hand grip strength and long reach to be able to turn the water on and off.
A toilet at the right height. Most designed to be easy for a disabled person to use are ugly as fuck and don't have lids, which is disgusting too high to be comfortable/poo freely if you're not a six foot two bloke. I'd love to have a washlet/bidet contraption in the toilet.
A thermostatic control for bath (preferably a whirlpool type and very deep) and all showers. Taps, too, would be great, as you'd know exactly what temperature everything was going to be.
Upgraded water flow/pressure from the outset. It's the main reason why boilers and showers don't work brilliantly. A boiler where the hot water comes on instantly when you turn the tap, whether the heating is on or not.
I wouldn't bother with the solar water, as I've just had my 6 year old one removed because it was useless.
Alternatively, a mixed fuel range that drove heating and hot water, but would work on wood or anything else that's burnable. I love ranges. And chimneys/fireplaces.
A second shower room with level access, somewhere to sit and dry off - but not a full wetroom, as they flood and become far slipperier than you'd expect. Possibly almost like having a slight sloped recess to where the shower is, so all water flows down, rather than mostly down, but tons all over the rest of the room. A large screen/door that opens to double width would also catch most of the splashes and make it possible for a person restricted in mobility to use it freely.
A large utility room. Space for large washer, large drier, both up on a special cabinet with drawers underneath - so you could store washing materials underneath, even if you buy large boxed powder and conditioner - and so that you aren't bending down to load and unload them.
Another vote for a larder that is cold, rather than a glorified cupboard in the kitchen.
Already got all power sockets (except for a couple in the kitchen units for appliances) up at chest level. Still need more in the kitchen, despite having 12 of the things. And at least two double points on every single wall in a room and at top and bottom of the stairs and hall/landing.
An overhang/porch type thing over all external doors. Not necessarily the prettiest of things, but far better than having the rain blowing straight in as soon as you open the door. Or a sunroom out the back, not a greenhouse like conservatory, one that can be kept cool as easily as warm.
Windows designed so that it's a doddle to replace curtain poles, blinds or shutters without needing to pay a professional to work around the weird sticky out bits in the house.
Measurement of all construction in nice, round figures. Preferably in Imperial because I'm old.
Top quality locks that cannot be broken in under 30 seconds.
Underfloor heating in kitchen and bathrooms. Massive amounts of insulation between it and the ground.
LED lighting that goes from daylight quality to mood lighting via touchpad, rather than millions of buttons that feel squishy and require a degree in electronics to understand.
Security. A wall high enough to stop kids walking on it and undesirables sitting on it. Preferably dry stone, as that makes it a vital nature reserve and you can grow spiky stuff on the top to put intruders off. A gate you control, so you can decide whether to let somebody in or not without them coming up to the door. A discreet IR camera that covers all potential entrance points to a resolution where they will be easily identifiable. And a low level one that enables you to view wildlife.
Windowboxes.
A wide and long enough garden that, rather than having fence panels to get blown down, you can have a proper hedge laid - one that is brilliant for wildlife and so damn spiky that nobody wants to take the chance of climbing through.
Trees. A waist height raised bed system nearest the house for herbs and cut flowers.
Rainwater harvesting. Large compost bins. Space for chickens. etc.
A proper depth to the soil. Some parts with sand in to improve drainage (growing spuds, woody herbs, carrots, etc). The darkest side to be more like woodland, so moist and fertile.
Bat and bird boxes, along with pollinators, inherently catered for in the construction. With camera access.
Its own supply of spring water would appeal. Or a stream running along the bottom - clean, gin clear, full of wildlife/fish/frogs/otters. No access anywhere along the course for flytippers, polluters or hunters.
Sufficient garden space for a small unit (stone, etc) that could be used as a place to care for and rehabilitate animals. Complete with heating, light (both natural and artificial), electricity and top notch security.