My drawing room front Window is going to be of around four feet wide. As the width of this room front wall is 10 feet out of which 3'6" will be kept for front entrance door. I am confused whether the sill height of the window should be 6 inches or 3 feet high as sofa is coming in front of it as my drawing room size is 10'0" * 14'0".

The comfort height toilets are typically between 17" and 19" to the top of the seat. The link below is a provides a nice overview of options to consider when purchasing a toilet. http://m.us.kohler.com/mt/www.us.kohler.com//us/How-to-Shop-for-Toilets/content/CNT800391.htm

At what height should we fix the granite slab (the height of the platform) to fit in a standalone washing machine and a dishwasher under the platform in the kitchen?

ideal floor to ceiling height is 10'.living room size 12'x16' or 14'x20' according to your requirement.