[zacherates]

Townhomes, duplexes, triplexes, etc are substitutes for single family homes (not for everybody, but for some people... like me). You'd expect that allowing them to be built would make single family homes more accessible not less as the people who currently are forced to buy SFHs but would be happy in a townhome would stop competing for them.

More supply of a substitute (in this case, townhomes) should result in lowered demand/price of SFHs.