Building a private jetty or pier at a Koh Phangan waterfront villa: the permit most buyers don't know they need

Want a private jetty, pier or swim deck at a bay-front villa? That isn't covered by a Building Control Act permit or the 2025 coastal eco-zoning rules at all. Any structure built in, over or under a public waterway needs separate permission from Thailand's Marine Department (the Harbour Department, กรมเจ้าท่า), under a law more than a century old: the Navigation in Thai Waters Act B.E. 2456 (1913).

The legal basis: Sections 117–118

Sections 117 and 118 of the Act prohibit building, planting or placing any structure over, in or under the water of the sea, a river, canal, swamp, lake or reservoir that forms part of a navigable waterway, without the Harbour Department's prior permission. The Department of Marine and Coastal Resources' own guidance states plainly that any project encroaching on a waterway needs Harbour Department approval before construction starts — not only large commercial developments. Violating the rule can mean a demolition order plus a fine calculated on the footprint of the unauthorized structure — Thai legal commentary on Sections 117–118 commonly cites a range of roughly ฿500 to ฿10,000 per square metre, though the amount applied depends on the specific case.

A separate regulator from the one you already know

The 2025 coastal eco-zoning (see Island eco-zoning and Coastal setback rules) and the standard District Office building permit (see Building a villa on Koh Phangan) both govern what gets built on land, up to the high-tide line. A jetty, pier, deck, breakwater or mooring structure extending past that line into the water itself falls to a different authority entirely — the Marine Department / local harbour master — and a land-side permit neither covers it nor is covered by it. A villa can be fully compliant on every zoning and building-permit front and still be sitting on an unauthorized structure the moment a private jetty crosses the waterline.

Enforcement is active, not theoretical

This isn't a dead-letter law. Reporting on beachfront and waterway encroachment cases in other Thai tourist provinces in recent years confirms Harbour Department jurisdiction and the per-square-metre fine mechanism are actively applied, including demolition orders against unauthorized structures — not just written into old statute. Koh Phangan's bay-front and hillside-with-private-access villas are exactly the kind of property where an owner might add a jetty or deck after buying without realizing a second, entirely separate permit track applies.

What to do before you build

• Confirm with the local Marine Office / harbour master (the Surat Thani provincial Marine Department office has jurisdiction over Koh Phangan) whether your planned structure needs Section 117 permission — before you build it, not after.
• Get this cleared alongside, not instead of, your standard District Office building permit and any coastal-zone compliance check.
• If buying a villa that already has a jetty, deck or seawall extending into the water, ask for proof of Harbour Department permission as part of due diligence — an unauthorized structure is a liability you'd be inheriting, not a bonus.
• Budget time for this separately — it's a different office, a different application and a different timeline from the land-based building permit.

A sea view is one thing; a legal way to touch the water from your own plot is another. If villa plans include a jetty, pier, private beach deck or breakwater, treat the Marine Department application as its own line item in due diligence — not an afterthought once the villa itself is built.