Hallway - Large cupboard to store shoes, coats, golf clubs, football boots and gear, bags, umbrellas, etc
Downstairs toilet essential. I would make it a shower room that could be accessed from the outside of the house particularly if you are sporty. Great to be able to walk in and shower without straggling through whole house.
Utility room - Don't skimp, go for good size. Great to have somewhere to put wet clothes on a rainy day. Make it big enough so that you could iron in it. A press large enough to take brushes, hoover, large washing power packets, shopping bags, laundry basket and other bumph
Kitchen - I would combine kitchen and dining room. Underfloor heating in this room if tiling floor. Decent appliances. Big larder for all food. Make room for informal eating such as island and then table for formal eating.
I would put all bedrooms en suite and skip a main bathroom.
Large walk in hotpress.
I would build wardrobes into the design of the house as they used to do years ago and not add them later, ie, shape the walls of the house to incorporate wardrobes and add doors. Saves thousands.
Put electric cable to back and front garden when building house. Put water pipe to back and front garden when building.
PLUMBING, PLUMBING, PLUMBING is very important. Get a good plumber. It can be disastrous if this goes wrong.
Power showers - need extra pump and larger pipes but worth it. Needs to be done at start.
Insulate and then insulate again.
Make sure back garden will get sun.
Telephone sockets in most rooms.
Playroom/Den for young children downstairs with plenty of storage. Cheaper to do at start.
Spotlights are a pain and are expensive. Keep your lighting simple with lamps.
Think where you are going to put your furniture in each room and plan your rooms around it.
Oh I wish it was me building!!