The North has the better beach, the South has better resorts

Sai Khao Beach South, or White Sands Beach, is the narrower and shallower southern end of this long, 'first' beach on Koh Chang. The beach here is completely lined with low-rise beachfront resorts, with those being generally small and budget to the top of our beach map, and getting larger and more luxurious as one goes South. The road runs just 30 metres behind the beach, squeezing all resorts into a relatively narrow strip.
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The top end of our South map – the middle of the beach – has a series of smaller bungalows and resorts. These are among the first arrivals on the island and left-overs from the old backpacker days. Some have upgraded, some still offer very basic, cheaper rooms. All of these open right onto the sand under the big beach trees. What they lack in star-rating they make up for in atmosphere. Our photos of this beach at sunset were mostly taken in this zone.
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At the south end of Sai Khao where the sand runs out, the shore turns rocky as it rises to a low headland. The last bit of the beach here is not the best, but the resorts are. These larger, more luxurious resorts share both sand and rocks. The two very long hotels here have built walls for protection from the sea and to give guests elevation and a view over the ocean and beach. These larger resorts have gardens, trees and a spacious green atmosphere, a big contrast to the crammed-in feel of the smaller hotels at mid beach.
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Koh Chang Grand View Resort has a full 300 metres of oceanfront spanning both the end of the sand and the start of the rocks. After this is Chang Buri Resort, another large, luxury hotel with over 200 meters along the rocky coast, with views looking up the beach. The two hotels beyond these have the word 'cliff' in their name for a good reason.
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This website offers info pages about virtually every beach in Thailand with a true beachfront hotel, and a page on every beach here in Koh Chang. Some of these might prove helpful:

White Sands Beach, important beach topics: sand, water, rocks

This beach earns its title White Sands Beach, the direct translation of its Thai name. All of the sand up and down this beach is fine and soft underfoot.
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The central section of sand is so wide that a good area remains dry at high tide. This is the only beach on Koh Chang with enough sand to remain dry at high tide. Only some spring high tides that follow the full and black moons can wash the entire sandy part of the beach. Most of our photos of a wide sandy beach are taken in this area.
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At the south end the sand gets narrower, the water shallower and finally rocks take over. The sand is still fine, but here it is harder packed and a little greyer than in the North. The water is still clear, but it generally takes a good walk to find enough of it for swimming. And at the very end of the beach swimmers need to watch the rocky sea floor.
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This end of the beach is not suited for serious swimmers. However, the beach here is just as relaxing and pleasant, and for the majority of guests the overall atmosphere and the luxury resort facilities are more important than the water depth.
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As with all of Sai Khao beach, there are quite a few people on the beach in high season. This is, after all, Koh Chang's most active beach with the largest number of resorts and people.

White Sands Beach: big shady trees, restaurants, bars & massage below

Trees give all of Sai Khao Beach its distinctive, easy-going lifestyle feel. The beach trees reach out far over the sand, creating a belt of deep shade that runs virtually the entire length of the beach. Resorts have expanded out under these trees, setting beach lounges and restaurant tables and chairs here. Guests use this shady zone far more than any other part of the beach or the resorts.
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Massage beds are found the entire length of the shady belt. Restaurants and bars serve here day and night. Guests while away their days in this shade, which makes beach umbrellas unneeded. Only towards the end of the day when the beach cools down do tables, chairs and people emerge out onto the open sand. Life on Sai Khao Beach is lived in the shadows.
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As on most Koh Beaches, sunset is the highlight of each day. Guests are here for sheer relaxation, and by the time the light starts to fade, most people have done little except relax through the day. The falling of the sun into the ocean is the big moment that most come out to watch.
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beware non-beachfront hotels, road behind White Sands Beach

Sai Khao also has more non-beachfront hotels than anywhere else on the island, so beware. The advertising of some will surely forget to show the big, busy road their guests have to cross to get to the beach. A few that are in fact beachfront also have rooms spanning both sides of the road. When do we include such split hotels on our beach maps, but only if the main parts of the hotel like reception, swimming pool and not less than half their rooms are on the beach. Then we tell users about the other, non-beachfront rooms, allowing them to control which side of the street they book into. Beach is always best, of course, but across the road is always cheaper, and some prefer that option.

Four resorts on our two beach maps of Sai Khao have rooms across the road, with two of thos ebeing here on the South map: Kacha Resort and Chang Buri Resort . These meet the criteria above, with their rooms across the road being an annex of the main hotel only.
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Alternately, people looking for the cheapest rooms will be happy with one across the road, for the resorts sell those significantly cheaper. Also, aside from the non-beachfront rooms of these four member resorts, there are several hotels and guesthouses in the commercial centre, often competing against each other with cheap offers posted on the street in front.

White Sands Beach; ATMs, minimarts; banks, bikes, restaurants, pharmacies...

With only 30 metres between beach and road here, all of the shopping and conveniences are only steps away. Banks and ATMS, pharmacies, minimarts, shops, optometrists, post office, car and bike rentals, travel & ticketing – if a tourist needs it, you'll find it in the little township at Sai Khao. At the south end of the lines of shops a little strip of bars with hostesses is growing.
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This mini metropolis is entirely oriented to the tourism business. Before the arrival of hotels there was no local community in this area – there is no anchorage for fishing boats. Sai Khao commercial centre is the biggest of its kind on Koh Chang, with the most services, and has the only really busy shopping street on the island where you will have to watch out for traffic when crossing the road.

who has the longest best beachfront...?

The rather stylish Kacha Resort has a long beachfront, with 125 metres of sand. But the beach beach begins to get rocky here, and the resort with the longest beachfront, Koh Chang Grand View with its full 300 meters of oceanfront has that divided between sand and rock.
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by John Everingham