Sukhumvit is the main place where foreigners look for apartments for rent and condominiums for sale or rent. Houses are harder to find in and near the central business district and are much more expensive than in the Bangkok suburbs.
The skytrain is in dark blue, elevated expressways in red.
Zone D, from Sukhumvit 63 Ekkamai outwards, where the modern highrises end with old Thai neighborhoods, but large houses and some new housing areas start to exist
As with most roads in Thailand, the even numbered sois are on one side, and the odd numbered sois are on the opposite side.
Sukhumvit Rd. starts at the elevated expressway interchange by soi 1. It continues all the way to the Thailand Cambodia border (!) so that you need to be careful when you specify "Sukhumvit". The area of most popular interest is just the first few kilometers (miles) in central Bangkok, between Sukhumvit soi 1 and soi 71 by the Prakhanong skytrain station, which is the next to the last skytrain station. The last skytrain station is the Bearing skytrain station at Sukhumvit soi 107, but starting at Sukhumvit soi 71, this is a densely populated old Thai area with little expat housing immediately around the station. However, starting around Sukhumvit soi 103, you can quickly commute by taxi to many places in spacious suburban Bang Na, from the last three BTS skytrain stations -- the Udom Suk, Bang Na, and Bearing stations.
The Bang Na region is discussed in a separate section, Houses for Rent in Bang Na . However, here it's worth noting that the high end Bangkok Patana International School is located down Sukhumvit soi 105 aka Lasalle, the St. Andrews Sukhumvit 107 campus is right at the Bearing station, and Berkeley International School is a short distance from the Udom Suk station. The Bang Na Trad expressway goes between Sukhumvit sois 103 and 105.
On the opposite side of soi 1 is our Central Business District (CBD) Zone A Ruamrudee, Lumpini and Patumwan region, and south of that is our CBD Zone E Sathorn and Silom region. Zone A is purely condos and apartments, whereas zone E has some houses, too, but not many.
This section covers only CBD Zones B, C, and D. These zones are designated this way mainly due to traffic flow and their different environments. Sukhumvit Road has a barrier median with two one-way sides which significantly restricts right turns and U turns to only certain places. The traffic on Sukhumvit Road is heavy and jams whereby long waits are common. Due to a canal, access to Petchburi Road to the north is limited to sois 3, 21, 39, 55, 63 and 71 because of a canal. Due to government property, zone B does not have access to Rama 4 Rd. except a road parallel to the expressway.
Zone B is a densely commercial-residential region and tourist zone served by the Ploenchit, Nana, and Asoke skytrain stations, as well as the Sukhumvit subway station at soi 21. There are very few houses in this area, and it's mainly highrise condominiums, entertainment places, restaurants, some small modern indoor shopping plazas, and an assortment of shops. There are more quiet places on the southside even-numbered sois because most of them are dead end roads with no crossing roads going east-west. Places on the north side are more easily accessed traffic-wise from multiple directions.
Zone C is beyond the major Asoke intersection, and starts with tourist entertainment areas at soi 23 then quickly changes to residential and expat commercial places. There is much more residential development on the odd numbered side (which is the north side), and there is a web of back roads so that you can go east-west parallel to Sukhumvit Road on this odd-numbered north side. On the opposite side where the even numbered sois are, you can't go far east-west on back sois before you need to turn onto the main road.
Zone D is a transition zone where the modern highrises end and old Thai neighborhoods still exist. Near the skytrain line, there are some groups of big old houses out here which many foreigners like, but the newest houses in this zone are further out. As you get away from the skytrain line, you must ask yourself whether you want a house here or else just go to a suburb a little ways down the expressway outside of the central region.
As you can see from the map, Zones C and D are relatively vast in both dimensions, as the space between quasi-parallel Sukhumvit Road and Petchburi Road widens substantially as Sukhumvit bends towards the southeast. However, space is not all the story. The back sois between soi 33 and 55 are much quieter, especially around soi 39 and around Sukhumvit soi 49. The widest selection of homes is in this area. There are also many restaurants, kindergartens, and ecclectic shops scattered around this region. Zone D converts to more Thai-style as you get close to Petchburi Rd and KlongTan, but there are sections of Zone D near to Sukhumvit Rd. which are foreign spec, nice and quiet.
Thonglor 55 and Ekkamai 63 are main roads with a lot of modern little shopping plazas, nice restaurants, and many amenities. From Ekkamai Road, you can easily go north to many suburban business centers, and Klongtan 71 has quick access to the RamIndra elevated expressway.