Planning the water system before building a villa or development on Koh Samui

On a new build, the water system is easiest to get right while it is still lines on a drawing. Once the slab is poured, the pipe sleeves are where they are. Once the retaining walls are up, the flat ground for tanks has been allocated to something else. Once the garden is planted, every new trench costs twice. The sequence below follows the decisions in the order the build forces them, for a single villa first. Where a development differs, it is noted, and the differences are summed up at the end.

1. Establish which water sources the plot really has

Everything else is sized from the source, so it comes first. A plot can have up to three: a well drilled on the land, a mains connection where the network reaches, or both. The answer is rarely known for certain at the design stage, and the honest way to treat it is as an open question to close early.

• A well. Whether the ground gives enough water, and at what depth, is only known once a bore is drilled and tested. Drilling early turns the biggest unknown in the plan into a measured figure while the design can still respond. What the rig needs on site is covered in preparing a property for well drilling.
• Mains water. Find out whether a connection exists on the road, what it would take to bring it to the plot, and how dependable neighbours find it. Do not assume a flow rate that nobody has confirmed.
• Both. Two sources feeding one storage tank give the property a fallback. Whether the well is worth adding at all is weighed up in is a private well worth it for your property.

The build itself also needs water, for concrete, blockwork, curing and cleaning, before the permanent system exists. A well drilled early can supply the construction phase. Otherwise the main contractor usually arranges water for the site, and that should be agreed in the build contract rather than improvised.

2. Estimate demand from how the property will be used

Demand is worked out from the finished property, not from a rule of thumb per bedroom. A family home lived in all year, a pool villa let to groups, and a villa with staff quarters and a large planted garden can differ several times over.

• Bedrooms, bathrooms and realistic peak occupancy, not the average
• Kitchens and laundry, including any staff or service kitchen
• The pool: the first fill, and daily top-up for evaporation and backwash
• Irrigation, which on a planted plot can exceed everything used indoors
• Staff quarters, outdoor showers and wash-down points
• In a development, the number of units, how they will be let or occupied, and which phases are lived in while others are still being built

Two different peaks come out of this. The peak day sizes the source and the storage: how much water has to be found and held. The peak hour, typically early evening with every shower running, sizes the pressure pump and the pipes. Mixing the two up is how villas end up with full tanks and weak showers. The pool and garden are worth separate thought; see using well water for a swimming pool and garden irrigation from a well.

3. Plan storage: raw water, treated water and the reserve

Storage balances a source that delivers steadily against a house that draws in bursts, and it holds the reserve for the days when a source is down. Where the water needs treatment, it is common to store raw water first, treat it, and keep a second tank for treated water, so the filters run at a steady rate instead of chasing every tap.

Settle three things on the drawings: how many days of reserve the property should hold, where the tanks stand, and what they stand on. A thousand litres weighs about a tonne, so a full tank needs a proper slab or plinth. Height matters too: a tank on higher ground can feed the house partly by gravity, and one below it will need a pump for everything. Leave access to every hatch for cleaning. Tank sizing and layout have their own guides: what size water tank a villa needs and designing a villa's storage tanks.

4. Leave room for treatment before the water test is in

Whether a well needs iron removal, softening, sediment control or disinfection is decided by a laboratory test of the actual water, not by the plot's location. That test often arrives after the building is designed. The answer is to reserve the space anyway: floor area for filter vessels, a drain that can take backwash water to an acceptable place, a power point, and enough headroom and door width to change media later. A plant room with no drain is the most common reason a good filter becomes a messy one. See testing well water before choosing treatment.

5. Plan pressure across the whole site

A booster pump turns stored water into pressure at the taps, and it has to be sized for the peak hour. On a flat plot that is mostly a question of flow. On a hillside, height changes everything: every ten metres of elevation is roughly one bar of pressure gained or lost. A villa below its tanks may need pressure reduction on the lower floors; a villa above them needs a pump able to lift the water and still deliver pressure at the top.

Pipe sizes deserve the same attention, because buried pipes are the hardest part of the system to change. An undersized main to a far bathroom cannot be fixed later with a bigger pump. Why villas lose pressure, and how much of it is pipework, is covered in low water pressure in a villa.

6. Plan the electrical side together with the water

Pumps are among the largest motor loads on a villa, and their power, controls and protection should be in the electrical design from the start, not added to whatever board has a spare way. That means dedicated circuits for the well pump and the booster, a sensible place for the control panel, ducts for the cable out to the wellhead, and wiring for level switches and any alarms or remote monitoring.

Decide early whether the water system should keep running in a power cut. If it should, the pumps need to be on the circuits a generator or backup supply feeds, which is far simpler to design in than to rewire. The trade-offs are covered in well pumps, power cuts and electrical protection.

7. Give the equipment a room, and give the room access

Pumps, pressure vessels, filters and control panels need a dry, ventilated space with a drained floor and enough room around each item to service it. The door has to be wide enough to carry the largest vessel out. The wellhead needs open sky or a removable cover above it, because replacing a submersible pump means lifting the whole column of pipe straight up. A wellhead built under a deck, a roof or a fixed planter turns a routine pump change into demolition.

8. Fix the water decisions before these construction stages

Each stage of the build closes off some options. The timeline shows what needs to be settled before each one, so the water system goes in with the building rather than around it.

When water comes after these stages instead of before them, the outcome is familiar: trenches through a finished garden, tanks wherever there happens to be room, a plant room without a drain, and a wellhead nobody can reach with a lifting frame.

Single villa or multi-villa development

The sequence is the same at both scales. What changes is that a development's water becomes shared infrastructure, with a capacity, a failure mode and someone responsible for it after the units are sold or let.

A development also has to choose between central plant and plant per villa. Central plant, meaning shared sources, bulk storage and a distribution network, keeps water quality and maintenance consistent but needs a clear owner after handover. Plant per villa hands responsibility to each owner but multiplies equipment and tends to drift into uneven standards. A common middle path shares the source and bulk storage and gives each villa its own pressure set. Whichever is chosen, it should be written into the sales and management documents, because buyers ask where their water comes from and who maintains it.

What the architect or project manager should receive

• Source information: the drilling log, yield test and water test where a well has been drilled
• The demand estimate, with its assumptions written down
• A schematic from source to taps: tanks, treatment, pumps and controls
• Floor space, weights, drainage and electrical loads for each item of equipment
• Routes for buried pipes and cable ducts, and the access needed to the wellhead
• A handover list: drawings of buried routes, settings, test results and a service schedule

Planning a new villa or development? Send the site and project details before the water infrastructure is fixed into the build. We can test the source, size the system against real demand and provide the drawings and specifications the build team needs. See villa water systems and deep well drilling, or send the project details.