Eric - if you don’t have a room to convert, where are you building it?
Do you have a basement?
Kris and the others gave you some things to think about.
For some people building a cellar is a really big deal and they even blog about it. If you grew up in the building business, it’s just an insulated closet that you do on Saturday afternoon.
If you’re hiring the work out, it’s a lot more expensive than if you do it yourself. But if you have no tools and you’ve never built anything, it’s probably worth hiring someone.
Then people pay a lot of money for the finishes - racking, etc., because that’s what they see. Actually they don’t - it’s full of bottles. But you can spend a lot of money on your racks and finishing touches.
Or you can do like I did and illuminate it with a bare bulb and build your own racks or bins out of 1x8 or 1x10s from the lumberyard. If you make bins, square or diamond, you maximize your storage space, but they’re inconvenient if every bottle in the bin is different. So depending on the racking, two cellars of equal size can hold very different numbers of bottles.
Figure that a Burgundy bottle is about 3 1/2 inches diameter. If you have individual racking, it’s about 3/4 inch wide, so for each bottle, you need 3.5 + .75 + .5 inch to give yourself some clearance, so you have about four and a quarter inches per bottle for length and width of your cellar. That’s not perfectly accurate because if you do individual racks, the vertical distance is a little different, but it gives you a rough idea. In bins, you save a lot of space. In a 10 X 5 ft X 7 ft cellar, that can be a difference of a hundred or more bottles.
While bins are great for efficient use of space, they suck for Burg/Rhone shaped bottles and they’re deadly for Turley/Pax/Beaujolais/fat ass bottles that don’t stack.
And your cooling system will cost you both to install and to run. So if you have any possibility of passive cellaring, that’s worth considering.
I built my own door, my own racks, and my own cellar and it cost me around $1000 for a space that holds 2000+ bottles. But I also grew up in the building business, have wired countless houses and offices, rebuilt my entire house myself, and have a table saw, mitre saw, routers, drills, and plenty of tools. For my shelves, I bought some plywood, ripped it, put a few coats of polyurethane on, and built shelves. If I had paid someone to do what I did, it would be several multiples of what I paid and that’s just for the most basic functional, stripped down cellar.
So a lot of your costs are going to come down to what you’re able to do and the final product you want. A good finish carpenter is a highly-skilled individual and if you want the work and can’t do it yourself, you’ll pay for those skills just like you pay a lawyer or dentist.