A year or so back I did a number of postings on “Wargaming for Grownups” about building a wargaming shed in the garden then kitting it out. I then turned that into an article in Miniature Wargames earlier in the year. The story starts here : http://wargaming4grownups.blogspot.co.uk/2012/11/what-does-wargamer-do-with-redundancy.html and then there are updates every week or so.
Steve’s got most of it. I don’t have a de-humidifier and as long as the insulation is good you only need a fan heater or two to keep it warm. Carpeting the floor makes a real difference if you’ve got some old rolls in the loft or similar.
Don’t forget to put in a tea/coffee making area. As for storage under the table…that’s a matter of taste. I was going to do that but like having a table where I can alter the size so we can all get round it.
Do NOT skimp on the lighting.
Oh, and have a hat shelf as well.