The standard mansard roof design features a steep pitch on the sidewalls (matching on all four sides of the roof) with a second shallower pitch towards the top of the roof. This creates an attic with a lot of useful space, which is why the style is often used in a modified form for dollhouse attics. The roofs often feature dormer windows, with Traditional French style second empire buildings often having arched dormers or dormers with rounded windows, known in France as Oeil de boef or 'bull's eye' dormers.
For dollhouses the mansard roof often consists of slightly angled sides with a flat top, avoiding a more complex construction. These roofs give a maximum room area under a roof, and can be easily hinged or have roof 'gardens' and decks set on top. A Field Guide to American Houses by Virginia and Lee McAlester, describes five different mansard roof styles: straight, straight with a bottom flare, concave, convex, and s-curves.
This photo of the 'Custom's House' or Malahat Building in Victoria, Canada, shows a classic civic building built of stone and brick, with simplified versions of many basic features common to second empire design. The mansard roof on this building features a bottom flare, and has a roof deck to overlook the harbor. Missing from the design is a crested railing, which was often found on mansard roofs, especially ones with decks, where the railings often enclosed a 'widow's walk'. The building has corner quoins, arched hooded windows, bracketed eaves, a slate roof, and brick and stone detailing.
The details of this simplified 'second empire'building are often seen in dollhouse designs for 'town houses' or French mansions.
The second empire styles with their useful space under the mansard roof were also very popular for civic buildings and institutions. Victoria City Hall designed by architect John Teague, was constructed in Second Empire Style between 1878 and 1891. It features a metal mansard roof, pedimented dormer windows, clock tower, arched windows and doorways and quoins.
Second empire style houses in the U.S often feature mansard roofs with a central tower or offset tower. This house style was used by Alfred Hitchcock for the house in "Psycho" and was also the style seen as the mansion inhabited by the Television "Adams Family" Mansion. Disney use this style of house for their 'haunted mansions'. Wikipedia claimed that the use of this style for 'haunted' houses may have its origins in the Panic of 1893 in the United States, which led to the foreclosure and abandonment of many then-new houses, built in the second empire style.
Great references for mansard roofs and buildings suitable for dollhouses and miniature 'haunted' houses.
Printables For Miniature Mansard Roof Houses and Decorations
Dollhouse Designs Featuring Mansard Roofs This is not a complete list
Dollhouse Plans Featuring Mansard Roofs