At the stage of initial construction of the house plan in advance that in the future it will continue. Roof structure is just so that truss system reserved space for the device the second floor (mansard type).
Wooden bars 50x50mm and 50х100мм make the frame. To do this, sew them nailed to the vertical posts of the trusses. Outside, from the attic, panel lining, waterproof sealing, and secure the insulation. Inside lay the vapour-barrier foil, make a crate and screws fasten it to the drywall sheets.
Then mount the ceiling. For the convenience of the work undertaken and part of the roof it is desirable to remove. The ends of the floor beams attach to the upright trusses. With the lower edge of the reinforce bars are 50x50mm and place them on the Board floor. On Board lay the layers on top of vapor barrier film, a layer of insulation, waterproofing. Hem the bottom of the ceiling sheets of drywall or plywood. External, removed roof replace on.
Now you need to make the natural lighting of the second floor. To do this in the front make the window opening, and insulate the wall from the attic. If the light is not enough, you can also make a roof lantern type over an alleged second window. You need to do the roof gap vertically with width of 100-120cm. The height of the gap provide at 40-50cm from the ceiling of the attic. In the wall make the window opening.
Make the floor the same way as was done with floor the first floor. The only difference is that on the second floor subfloor is not spreading. At the final stage determine the installation location and design of stairsthat will connect the first floor and attic. To save the occupied area, use the spiral staircase.