Whether you are on city water or well water, have a water quality test done. This is an inexpensive, easy procedure and may save you a lot of money in the future. Private labs or your county Extension agent can help you take the water sample and have it analyzed. Check to see what the level of soluble salts is and the level of bicarbonates. Low soluble salts, a level below 0.75 mmhos, will be best because you will be adding fertilizer to the water during irrigation.
Some salts are toxic to the plant when they are in excess amounts.
The level of bicarbonates will be important to some types of plant production, such as plug production. You will also need a plan for water collection and you should plan for water run off from your greenhouse. You will need electricity and, depending on other equipment and needs, you may need gas or water or sewer services connected. Check with each supplier to determine costs and to anticipate any difficulties they might have in supplying your services. Local building codes.