Here is a short primer on building in Belize.
Unless you are planning on building a 1 room shack, you will need a building permit to start construction.
Non-commercial buildings under 900 sf do not have to have an architect's plan, but will need a "technician's" plan. The architect or technician need to be from Belize, and you can contact one once you arrive in Belize. It helps the process if you get an endorsement letter from the Village Council too.
The plan will need to include a site plan, GPS coordinates and indicate landmarks, septic location, plumbing diagram and electrical plan. Your technician or architect will already know this and ask you if you want to have them include it in the plan. Just say yes.
After applying and paying for your building permit, in Belize City, it will take a couple of weeks for them to "review" it, even if they have no other projects ahead of yours. Once approved they will want to come out and inspect the staking, etc. You will go thru a couple of inspections during the building process and will hopefully finally receive a certificate of occupancy, which is necessary if you ever want to sell the property.
To apply for an electric meter you will need to have used a licensed electrician, licensed in Belize. Right now, Caye Caulker only has 1 licensed electrician. He will provide you with the signed form that you will need to use, you will bring that, along with a copy of your land title or something showing that you own the property, to set up an electric meter and service. You can set up your electric service in San Pedro or Belize City. The Caye Caulker office only deals with technical matters, and not with accounts. Plan on the process taking 2-3 weeks.
Your water meter is a bit easier. You can go to the office on Caye Caulker located near the back municipal pier on a Tuesday or Thursday, when they send a CSR from Belize City. The water company technician has to come out and do a site survey to see where the existing hook ups are. You will be given a meter number and an account number within a week. You will need to go to Atlantic Bank and pay for your meter, installation and security deposit. You will take your receipt to the water company office on Caye Caulker and then will schedule the meter hook up.
If you use a Mennonite builder, they have their own plumbers and electricians available, and can finish the inside too, however their choices in interior design are quite dated to say the least. I have used local Spanish builders with local electrician and plumbers, and I have also used the Mennonites to build. Its a mixed bag, I couldn't say which I liked better. You will definitely have to be here EVERY DAY during the process, and contacting people in advance while you are still in the U.S., even though it reassures you, it doesn't make the process go any faster once you are here. In the past, the Mennonites have gotten a bad rap for bad quality construction and wood, but frankly, the Spanish builders are buying their wood from the Mennonites too, and they have brought their quality up a lot since I moved here 15 years ago. You can check out pricing and floor plans at the Pletts website, along with pricing. I have never built in concrete, only in wood. The rules for building in concrete are pretty much the same. I hope I have answered most of your questions. And maybe in years past they have shipped completed houses on the barge and just set them up, but the Mennonites are not doing that any more. There are only 2-3 Mennonite companies that will come to Caye Caulker. Tropical Homes, Pletts and Tuarg (sp?) are the 3 I know of. Pletts has the most complete website, and was the only Mennonite builder to consistently answer their phone and emails. Tropical Homes does beautiful work, but I didn't have the time to wait around until they decided to get back to me.
Ambergris Caye 
AmbergrisCaye.com