About the cistern...

I do not know much about cost or construction of a cistern. However, the minimum size might need to be reconsidered unless it is for a small efficiency type cottage.
A minimum size cistern for a family size home would be frustrating and expensive to be running out of water and purchasing expensive truck loads of water routinely.
In my opinion, an extra large cistern and efficient water collection would be beneficial for everyday living and an advantage to a potential buyer if you eventually sell the property.

Cisterns are typically part of the house. Sometimes they are buried and sometimes they are on ground level. In either case the main living space is built on top. So the cost would be included with the cost for the entire structure. There are formulas for the amount of roof surface required to collect x amount of rain based on the size of the house. That will dictate the minimum size of your cistern. Many people build two cisterns. One advantage of this is if there's a probelm with one - say a leak - you can switch to the other while making repairs.

Standby as I'm sure more folks will chime in with additional info.

Most cisterns are built under a home.
Majority of homes are not connected to public water.

It depends on size of home you're building.
Majority of homes outside of town areas depend on rainwater collected from roof area and deposited thru downspouts to cistern.

I'm sure there is a building code formula but I don't know what it is.
More is better but contact DPNR regarding building codes for your lot/zoning and discuss with your architect.

You will be totally dependent on rainwater.
I have a 16000 gallon cistern.
We had a severe drought period last year and with tenants in residence, it was their 1st time I'd ever had to purchase water in decades. It sucked.
One learns to conserve and not waste water.

Cisterns are typically part of the house. Sometimes they are buried and sometimes they are on ground level. In either case the main living space is built on top. So the cost would be included with the cost for the entire structure. There are formulas for the amount of roof surface required to collect x amount of rain based on the size of the house. That will dictate the minimum size of your cistern. Many people build two cisterns. One advantage of this is if there's a probelm with one - say a leak - you can switch to the other while making repairs.

Standby as I'm sure more folks will chime in with additional info.

Right, the cisterns would be an integral part of the foundation, in lieu of a basement and could be compartmentalized to allow for more than one as noted. Our condo cisterns are cleaned and resealed every few years as part of routine maintenance which makes having more than one a plus if not a requirement.

Size will base based on square footage of first floor and height of cistern walls. Depending on size of home, terrain, size of cistern needed the cistern part may occupy less area than the floor above.

I'd expect there is a calculation for minimum cistern size based on number of bedrooms similar to how size of septic systems are determined. Number of bedrooms == number of occupants == total consumption.

Besides cost and size of cistern (a topic that comes up frequently here) you'll need to factor in a septic system whose size is also determined by number of bedrooms / bathrooms on the property. A good Architect and lots of pre-planning is essential. Really take some time to come up with a solid, complete plan, it will save $ in the long haul.

I agree with others, you don't really have to figure out the cost to build a cistern as it is usually part of the foundation.

When you have your initial talks with architects and contractors as you research the cost to build your house you'll be given estimates based on a cost per sq. ft. For example, they might tell you it will cost you $300/sq. ft. to construct a home using builder's standard materials or perhaps $450/sq. ft. using designer finishers, or even $225/sq. ft. for just the foundation, walls, seething for the roof and openings for windows and doors with you supplying all the finish materials separately. At any rate that cost per sq. ft. will include the construction of the cistern.

Now, the thing you really have to worry about is cost overruns and they can mount up quickly. I know scores of people, including me, who have built homes in the Caribbean and not one of them has come anywhere close to being at or under the original estimate. We were behind schedule and over budget a week after breaking ground and those gaps continued to grow until our house was finished.

Best advice, get your best pre-construction estimates and add at least 30% (50% would be better) to that number. If that revised number doesn't scare you away proceed with the project.

Good luck - you are embarking on a labor of love!

30% - 50% sounds rather high.
We had a few increases, but the majority were due to upgrades as we went along. We decided on a tile upgrade after our inability to find anyone on island who could pour a finished cement floor with color additive. After reviewing counter top options we decided on granite, but that was also not our first choice. Added Custom Fees for windows and door manufactured in St. Martin and some other shipping fees bumped us up as well. In the end I don't think we increased more than 15%.
Took two years from planning to compleation though.
Maybe we just got lucky?
However, finished product was spot on.

Also very important to the overall succeess of your project...and I believe you addressed this in your original post...BE ON ISLAND! Having a house built while off island is a formula for disaster.

My advice is to plan your cistern carefully. Consider the location of the lot on STT and the rainfall in that area when you are deciding on size, make the access hatch or hatches useful and have 2 compartments either with a dividing wall or completely separate. If it is just one compartment make the smaller one the one that receives the rain from the gutters and the other one the primary use - that way the smaller one is always full and can be used as a reserve until it overflows again into the bigger one after some rain and some increased conservation if you are on reserve. If the access hatch is placed properly you will also be able to clean one if the other is empty with little disruption to life.
A smaller gray water cistern that fills from the driveway and is only dedicated to toilet flushing and irrigation is a very good plan in the drier areas out east.
Your cistern is really an important functional part of life here and you will be Much happier down the road if you think it out ahead of time.
I have been in my house 36 years, never bought water but live in a very fortuitous rainy location.

If you planning on having tenants make sure you have at least 20000 gal main cistern with backup 10000 gal. My tenant left for a week leaving leaking toilette effectively draining 20 000 gallons in 7 days.

Minimum cistern capacity for dwellings in the VI is as follows:
10 gal per sf of roof area for single story.
15 gal per sf of roof area for two or more stories.
4.5 gal per sf of roof area for accessory bldgs.

There are lots of ways to lay a cistern out. There are 7.48 gallons per cubic foot, so an 8' deep cistern would take up about 17% of the footprint of single story houses or 25% of the footprint for multi story homes to meet minimum size requirements.

Cost will vary depending on size and layout but if you do a lot of the work yourself it can be done for about $1 per gallon.

SpecialK,
I think your right.
During construction, we were back and forth to STX every 2-3 months. It was the best we could do back then. I'm sure things would have moved along a bit quicker if I was there full-time though.

If you planning on having tenants make sure you have at least 20000 gal main cistern with backup 10000 gal. My tenant left for a week leaving leaking toilette effectively draining 20 000 gallons in 7 days.

Probably cheaper to just invest $1600 on a whole house water leak detector like the Flow Logic. In addition to shutting of the water in case a pipe bursts or appliance leaks it will detect just a few drops leaking from a faucet or toilet valve. I've had one in my house for over 10 years. Works like a charm.

. My tenant left for a week leaving leaking toilette effectively draining 20 000 gallons in 7 days.

No one heard the water pump running?? You can avoid that in the future by explaining they should throw the breaker for the water pump if they leave the hose unattended for that long again. Even an outside hose could split and do the same damage.

As an aside, I thought that new construction had to have sewage treatment instead of septic?

I believe you're right, EE.