mahan
If my guess is right, you might have seen those houses in SMV layout (in 3rd or 5th Stage), where plenty of such houses are built and put up for sale specifically made by the contractors. Here are some of my opinions
1) In a 20X30 site, nobody follows setback rules in leaving space either on left or right or backside. Even though its a violation, think from the point of availability of natural Light and Ventilation. U need to depend only on the roadside, which is hardly a 15 or 20 ft wide.
2) Since the Road width is small, Car parking and taking a Turn is a tedius & risky.
3) Sanitary and Water & any other amenities comes within this small 15-20 ft road and in case of any BBMP/BWSSB/Telecom repair or relaying, the whole road is literally unusable for couple of weeks or months. Also the electric lines passes very close to the houses(near to balcony or Windows), which is a risky to the occupants especially Kids.
4) Since the road width is small, many people doesn't have option to have Road trees on eitherside. In many cases after some period, the road will be concretised and you need to bear the reflection heat in the summer season
5) 3 BHK in a 20X30 site means, Car Parking + Sump + Borewell in the Ground Floor, 1 BHK + Pooja room in the First floor and 2BH in II Floor. Since the availability of length or width is minimal, the steepness of the staircase is usually both from Ground to First and First to second floor and especially if you have senior citizens or people under medical supervision, it is not recommended.
6) If you would like to have a borewell/well, its only in the basement area/alongwith the parking, which is against vasthu.

. Also implementing RWH is bit challenging.
If you are having a budget of Rs. 50 lakhs and above, better to go for a 30X40 plot size or an apartment with No shared/common walls to have better satisfaction rather than 3BHK in a 20X30 plot size.