I sadly had to move to a ground floor flat from my precious 3 level garden in North Yorks, and miss my lovely summer house /shed very much . First thing to do is sit with a bit of paper and write down what you want from your place, what you are likely to use it for. Is it going to be your place or shared? Then when you have done that start noticing places round about you. Have you seen a garden that has a style you like? What do you want to do in your own garden. Have you a greenhouse already or nothing .Do you want to be close to your house - easy to go in and out - or well away to become your private area. So I am a keen gardener, with little money , so it had to be right first time. So after looking round, seeing things that I liked and where possible asking the owner where they got it from etc, we were able to go to a great place, where they made summerhouses. You could go into this large barn where there were lots of different ones to look at., so that you saw exactly what they would look like in your place. We ended up with one of their sturdy summerhouses but modified to my choice. So the ordinary one had one back solid wooden wall, two large side windows giving the best light and at the front was a double door so that you could open one or both doors. I got them to put a long narrow window in the back wallto give more light , and then they came to erect it on the lowest of my three level garden, , well supported and we added paving outside later. I have a bad back so then I was able to furnish as I wanted. We put two comfortable chairs and had a couple of fold up chairs if friends came by. Had a sturdy small wooden table and have two gardening encyclopedias, so the old one lived down there. This enabled me to check up or look up things without having to go back to the house. Had my radio (usually tuned to radio 3 or 4 , for plays and gqt etc. Then we had a two way walkie talkie to be able to check when dinner was ready or needed some coffee or whatever. There was no mobile phone there I didnt want it anyway. I kept an old Mac and an old fleece there so that if I was gardening or the weather changed could just go in there. Also had hats and gardening gloves all at the ready. Marvellous in the autumn and winter , as you could have a lovely time there reading or whatever, watching the frogs in the pond, cosy and warm , even if the wind was wild. Having gardened for years have duplicate tools etc and kept a whole lot down there and my binoculars to watch birds or whatever. had the bird book down there too. Having a bad day and need some private time, go down to the summerhouse. Having a good day invite a friend and out come the glasses and wine. Could sit potting up plants, taking cuttings etc. Weather good open the doors and sit outside, going chilly get inside.The large windows not only allowed you to really be part of the garden but also warmed it up too, and the double opening doors which were half window too allowed you to bring a trug or wheelbarrow in. Think about where you are going to put it. I wanted it in an area that suited me but did not take any of the best quality of soil etc . Wanted my cake and eat it . I managed to have it placed which was so private, yet looked over a lot of the bottom garden and had a lovely tree growing on the 2nd level which in the spring and summer hides it even more. I planted a katsura tree . Lovely heart shaped leaves, beautiful autumn colours and the leaves smell of candy floss!! Then the leaves fell and that meant more light in the winter. Sorry I am going on , but I felt every penny spent on it was worth while and it was one of the things that the person who bought my house really liked and was talking of exactly the same things. You need to be a bit like Goldilocks. Too small and you will regret it and not be able to pot up plants or have yourself a shelf of indoor pots there. Too big and it takes up too much garden and wont be as warm . Just right and you will be happy for years! Another tip is to just put a comfortable chair outside and then gather round it all the things that you would need to have with you, plants, knitting , sketch pad coffee cups etc etc. Collect them all in the manner you would want them in the summerhouse then measure the space and it will give you a basic idea of the size you would need.. I think if I was you I would allow myself to work out just what you would like to have, and make it yours . Go down to your local allotments and wander about and chat to people and ask them how they use their sheds. Not that you are going to do that with yours but gives you some ideas. I envy you the pleasure of the choice and making your very personal space!! Take pictures of it when it is done, plant things round it and take photos of them and write the date on the back so that over the years you will see how your place develops and becomes even more dear to you. I was a professional singer and used to write a bit so I had scores and pads and pens etc. I could go down there and not worry about disturbing neighbours and did not feel self conscious there. Oh lucky you !! I hope you find the one that is just right for you. My wooden building was more window than wood but used to put wood preservative on it each year and that was fine. Do let me know how it goes. Good luck.