Easier to stand it on paving slabs.
A wooden greenhouse is warmer, but has to be painted most years as the wood goes rotten.
The soil needs sterilising for Tomatoes, so maybe best to use Grow Bags.
Its not really possible to put plants on the shelf below the staging, not enough light, so keep that area for pots, and sundries.
Get double quantity of fleece to cover all plants in the winter months. Bulbaholic removes his every morning and puts it back on at teatime.
Get a greenhouse with an automatic opening vent for the hot weather.

Get the biggest greenhouse you can afford that will fit into the space - otherwise you will regret it in a few years! I agree with Diane slabs rather than concrete. What are you planning on growing in your greenhouse? We have one that is just for bulbs and has staging along one side and a plunge bed at ground level on the other. Our other greenhouse is for alpine plants that don't like the wet and this is all staging.

Totally agree with the others. Big as you can, you always find extra uses once it is functional. Slabs (or even loose breeze-blocks) are more practical. If you find something to plant directly into the ground (a grape vine for example) it is impossible if it is concreted but easy to take out a slab. Siting. I try to aim as close as poss. to south-west to north-east axis for mine. Take full advantage of what warm sun we get in the UK but as Diane warns, don't let it over-heat if we should get a rare hot summer.

As already said, always buy the largest greenhouse you can as it will never be big enough when you start to use it. I have a 8x10 feet coated aluminium greenhouse. I have dug out footings and placed it on two courses of brick. I did this because it needed something substantial as the amount of glass involved is very heavy and I didn't want the structure to warp and the house can be screwed or bolted down. I have double doors which are handy in a hot summer to keep the house cool. Auto top vents and side louvres help to keep a good air flow in hot weather. I have concrete under a potting bench on one side and concrete path.The rest is open soil which can be replaced from time to time to keep down the possible pests and diseases. I would suggest a south-west facing position.

A concrete base is for life and you cannot change your mind at a later date. I would fix the greenhouse into the earth and then lay a slab floor inside it. If, in a few years time, you decide to try growing directly in the ground then you just have to lift the appropriate slabs.

I bought a 12 x 8 last year, buy I opted for a single door version, as if you have a double door, what you usually end up with is a 3ft wide path and narrower beds or staging, with a single door you can have a 2ft wide path, and up to 3ft wide beds or staging, hence more actual growing room, for ventilation I chose to have 2 extra roof vents with automatic openers, and 1 louvre vent on each side, you can still leave the door open for extra ventilation should we get a decent summer, as for flooring, unless you're growing rain forest plants that need constant moisture and high temperatures, you would be better off with flags, or better still in my opinion, a layer of weed suppressing membrane and then pebbles or shingle, this would allow any excess water to drain away, and as the others have said, the largest you either have room for, or you can afford, hope this helps Derek.

Many thanks to everyone for all the useful tips. Lots of food for thought! May be I'll put the concrete on hold for a while, I like the idea of having the option to alter it if I want to, as in using paving stones which could be lifted if I change my mind.

I forgot to mention, the Alpine House at Kew has big blowers to get good air circulation, round the precious plants. April is the best time to go. Marvellous !