Greenhouse
Polycarbonate (at under $3/ft^2). A very rough estimate is that the glazed perimeter will be 14ft and the length will be 20ft - giving a double glazing cost of about $1,680. Considering a monthly vegetable offset of around $50, means that the green house (glazing) will have paid for itself in 3 years. The north wall can be wood or plastic, what ever is cheapest and most structurally sound.
In the center will be a large Koi pond, probably about 10x10x4. This serves as the reserve for watering the plants. The koi living in the pond will supply nutrients to the water. Water will be pumped from the pond to a small water tower, automatic sprinklers then just have to open a valve. Rain water from the roof will drain into the koi pond.
I also want a small patio area because I think the greenhouse will become a functional space, great for reading.
http://www.themodernhomestead.us/article/Greenhouse+Basics.html
Automation
Based on http://www.youtube.com/watch?v=KZiqUZGjGc0
The core will be an Arduino (or two) with either a wifi shield or connected to a RasPi (or something completely different - I'm not sure what will be available).
Water
The garden will be broken up into grids. Each grid will be a physical PVC grid (or manifold) with small holes to act as a direct soil sprinkler (cheaper version of drip irrigation). The size of the grid will be how large it can be while maintaining pressure. Each grid will be controlled by a solenoid. Each will have a soil-moisture detector (easy to make). When the soil moisture gets too low, it will send the central control a request. The central control will only activate one grid at a time (pressure).
Temperature
The pond should help stabilize the temperature passively. There will be a fan if it gets too hot. It would be nice to have some way to chance the thermal conductance of the pond (I'm thinking giant heat sink). I may also include heat-lamps or propane, but that would be for emergency frosts.
Size
Say I wanted to grow 50% of our food in the green house. I've always estimated about 2sqft/day for food. So 365*2 = 730. But I get a ratio of 3ft of moving space to 4ft of growing space. Maybe I can do a bit better with some vertical gardening, but calling 4/7, I need 1278sqft of space.
- 14ft (2G,3w,4g,3w,2g) --> 91ft
- 21ft (2g,3w,4g,3w,4g,3w,2g) --> 60ft
I think it makes sense to start with something like 7ft(2g,3w,2g)x15ft, which is only 30sqft of garden, but I can easily extend the length, and less easily, expand the width.