Hello Yaffle,
Grey Kitty told me he was sharing a link to my thread over here. Besides the farm, we have also been trying for 3 years to complete our half-built house. We have already had the money in hand once and my uBPDw burned through it before we could do anything. I took the last $5,000 or so and tore down the old house, boarded up the doorway from the old to the new, and laid the foundation. There it has sat since 2012, until I finally raided my 401k and put the money in an account in my name only. It is payable to her on death, but she cannot write checks or withdraw money on it while I am living. At my bank I could set up an account like this without telling her or needing her signature.
The reason it has taken so long to get to this point was that she has no concept of size, cost, or planning. We would go to bed with a solid, workable, affordable design that we both agreed on and when she woke the next morning it was like nothing had ever been discussed. Now we had to have two stories, a basement, a fireplace, a deck, a second floor screened room for the cats, a pool, a flat roof with a railing so we could walk around on it, a barn shaped roof, an attic, an arched ceiling, a sunroom, etc, etc. Everything I have listed and more has been on her "must have" list at one time or another.
It has finally come to the point that this must be finished or else. My insurance agent is warning that it can't be renewed in its present state. We have no permanent kitchen so it doesn't qualify for FHA or other financing, and I make too much money for any kind of rural housing assistance. Taking a hardship withdrawal was my only option, and if we spend it on anything else but finishing the house we're in trouble.
I told her how much I had, and that was
all there was, period. I told her it was in the bank in its own account. Remarkably she has never asked if she is on the account. The builder is a family friend, and I had him give me a flat fixed price to get it to a usable condition. My father and I will do plumbing, electrical, etc. There is a small amount left over for flooring and things not covered, and for the inevitable overruns.
When she couldn't run the show this time around, she pulled one of her usual tricks, which is to allegedly wash her hands of the project. Of course this is done with full intentions of immediately weaseling back in. I took her at her word and have proceeded as though she meant exactly what she said. For the most part this has worked, although there has been some whining and attempts to tell me how to run things.
The addition is essentially one large open room (1000 sq. ft.!) which will be a great room/kitchen. We already have a full set of kitchen cabinets and appliances in storage. The one concession I made when she really started in about "not being a part of building her own house" wa
s to tell her that when the time comes, we will bring all of the kitchen stuff into the room and let her place it exactly where she wants. So far this has somewhat pacified her. Because of the layout I was fairly safe to offer her that option, although I'm sure when the time comes she'll insist on putting the stove on the ceiling or something equally impractical.
I was fortunate in that I was able to control the funding this time around, which sounds like is not an option for you. Maybe offer something like, "You handle the kitchen and I'll handle the conservatory" and try to think of some way to manage the circumstances to keep her from getting control of the conservatory part? Is it feasible to do both at the same time? I'm thinking the kitchen project seems much more complex than a conservatory. I was just wondering if overseeing a major kitchen remodel would divert her enough to allow you to build a reasonable sized conservatory while she was distracted

As a general principle, anywhere you can find a place to set a boundary regarding the project, and stick to it, will help that much. With my wife if you give an inch, she'll take it all, so even small boundaries will help. I'm using our absolutely fixed budget as a justification for any decisions I need to get made without her dragging us off down a trail. Good luck--