From The Collaborative International Dictionary of English v.0.48 :
Semidetached \Sem`i*de*tached"\, a. Half detached; partly distinct or separate. [1913 Webster] Semidetached house, one of two tenements under a single roof, but separated by a party wall. [Eng.] [1913 Webster]From WordNet (r) 3.0 (2006) :
semidetached house n 1: a house with two units sharing a common wall [syn: duplex house, duplex, semidetached house]