Very little indeed. Remember this is a traditional village style center with twin-shared huts.
Usually guests leave valuables and smart clothes at home: virtually nobody wears them and termites may nibble them. The sarong you're given on arrival makes a wonderfully cool and comfortable dress, or a ‘kilt’ for either sex. Ulpotha also sells other sarongs in stunning colours. So people wear little else. Women need something arm-covering and leg-covering, for occasional cool evenings, and preferably in white or light colours, for visiting temples. You might also need 'uphill shoes' for walks or reaching the mobile phone spot and clothes for cycling or walking. Flip flops are useful as shoes are not worn in any of the huts or main house, and they are easier to slip on and off than lace ups. Rain is rare but, when it comes, it deluges. Ulpotha has umbrellas but shoes that tolerate drowning are useful. Ulpotha provides towels but a beach towel is useful too.