The group, which holds a portfolio of around 2,023 developable ha and 18 town centre schemes, said it would hold off selling any of its residential land until the market stabilised.
It said the "pace and severity" of the decline in the residential market had taken the company and the market as a whole by surprise.
In the six months to 31 May, St Modwen wrote off £37.7 million from the book value of its residential sites, as well as £16.9 million from its commercial portfolio.
Its pre-tax losses of £20 million compared to a profit of £65.1 million in 2007.
A company statement said: "We are reporting on what has been the most difficult period for the property industry since 1990.
"This means that, for residential land, there is currently no genuine market, as the major house builders are unwilling to invest in land until they can see the way ahead more clearly."