I am constructing my house and I excavate black cotton soil upto 5 ft, should I fill same black cotton soil as a backfill?
Finding Black cotton soil during foundation excavation is too bad, & technically it is the week soil bearing capacity due to its cohesiveness properties. i can say this much you better to meet structural engineer, get the structural drawings & work your house construction based on structural design drawings.
Structural engineer will do proper analysis & based on the structural analysis he will prescribe the adequate specification for foundation & superstructure RCC elements, & defiantly from which you will save more money in excess steel & concrete pouring inside foundation & also structural safety of your house will be ensured.
For structural Design drawings requirements you may contact HousePlansCenter.com. we will be happy to help you on this.

How do I plan a space efficient 2 BHK house on a 30×50 east facing plot?
There are many aspects to look like
Space management,
Requirements depending users
Locality/temperature/Rainfall intensity etc..
You may check out for similar plans &also ask for the customized plans in this link.

How long does it take Engineer/ Architect to design the exterior of a house/building (no interior floor plan)?
House plan design preparation is really not a give & take job. Architects will have their own time, they will spend in working for the client requirements in depth & purpose of plan to be fit to plot dimensions with proper space management by the engineering skills & knowledge of vastu concept in plan as-well in exterior designs based on which materials to be use & budget of construction. Expected duration 2days to 10days.
What are energy saving homes design? How are they made?
in generally, House plan designs should be made with sufficient natural ventilation to all the corner of rooms at interior of the house. so that saving of energy in terms of avoiding mechanical ventilation (AC, Lighting),

A best house elevation design / architecture of the house should made Provision to fix solar water , electrical panels etc utilising natural resources at maximum to be ensured with consideration of safety in all terms & having a good & decent landscape at house front or surrounding setbacks area.
In widely energy saving homes is huge subject consists of building embodied energy, alternative building materials, indoor & outdoor landscape etc.