There is an overall excess supply of housing in Bangkok, but it's concentrated in areas where people don't want to live. The 2013 zoning plan - which was a disaster - explicitly aimed to reduce density in the city center and push people toward the suburbs in Nonthaburi and Samut Prakan.
Reporting from the Bangkok Post and The Nation in 2012 warned about the consequences: higher housing prices in central Bangkok and worsening traffic in suburban areas as people were pushed out.
Developers built low-cost housing where the plan mandated: outside the city center. But, demand was for city living, because that was where middle-class jobs were.
Today, there is an excess stock of low-cost housing in the suburbs, while vacancy rates in central areas are low. Housing sales are down because access to financing is restricted, mostly but not entirely caused by monetary policy errors.
The new zoning plan attempts to correct these mistakes. It allows for higher density along designated corridors and opens up commercial development near new transit nodes. YIMBY reforms will - amongst many things - make it legal to build starter homes.