My top tip is to really consider what you want out of the kitchen and how you will use it.
Kitchen/Diners with islands and breakafat bars work for some, while others prefer a totally separate kitchen so they can shut the door on the mess and smells.
We have just totally relocated our kitchen from a back room to a side return (historic extension which was used as a games room previously). We now have a massive island (3.1mx1.5m) with breakfast bar,. We have space for a small, family dining table at one end and sofa and TV in the other end of the room.
It works for us as we are quite social cooks who like to spend time together in the kitchen, children doing homework/drawing/playing a game or adults sharing a glass if wine while getting a meal together. In the past we either ended up with all four of us cramped in one room or one isolated while the others were off in a different part of the house.
Slightly unusually, our hob and ovens are in the island with the sink under the window, this allows me (or whoever is cooking) to always be facing the others, I HATED the taught of always having my back to the family/guests.
My other top tip is to really consider whether you need wall units, as a PP said, they are often too shallow and can be very tricky to access. Our last kitchen set up had a lot of wasted storage space as it was just too high for me (5ft7).