You mentioned the prescence of peat at the site. Peat typically undergoes decomposition, resulting in large secondary compression (depending on the amount of peat at the site). One method (and it may be costly) would be to found the residence on piles that extend into suitable soil.
A thorough subsurface investigation should be undertaken prior to any attempt at designing a retaining structure.
A compentant geotechnical/civil engineer should be consulted and in charge of any attempts to retain the soil.
20 meters of peat is awfully risky material to build on. The amount of secondary compression (from decomposition) is highly unpredictable and is usually very high. Do you have any alternate foundation types from which to choose, such as piles?
