As per your question you want to construct a room with bathroom on second floor with tight budget. You may construct your room and bathroom with ordinary specifications i.e. Brick Walls, M-S Angle iron door and window frames, Hollock or Marandi wood door and window shutters, cement concrete conglomerate floor and simple plaster in room and bathroom with cement plaster with rendering etc.
 
As per your second questions you stated summer harassment at first floor. To overcome this problem thermal insulation provided to ensure heat insulation of exposed doors and windows, roofs and walls is given below.  
1. After making room at second floor the problem of heat from roof is minimized on the first floor.
2. The wall towards west direction reflects maximum heat in the room than the walls of other directions. So if you want to insulate just one wall to save money, you should target this wall.
3. For door and windows you can use insulating glass or double glass with air space which may be provided for glazed door and windows. This will reduce heat transmission through door and windows.
4. Protection in form of sun breaker, weather sheds, projections, curtains, venetian blind etc. may be provided on exposed door and windows.
5. For wall insulation you may fix hard board, ply, wooden battens, fiber glass, thermocol, rubber foam inside and outside to create an air space as these reduce heat.
Please let us know if you need further information.

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