To my knowledge there arent many DTCP approved layout before mahabalipuram on ECR.how will you get DTCP approval for your building plan for unapproved plot?Also, Approval of FSI more than 1.5 is possible only for multi storeyed buildings or by opting for premium FSI.
If you are buying a patta land and building a house on your own land by informing the local panchayat.Even if you deviate, no one will question until DTCP/CMDA rules start to be enforced.That time the plot is unapproved and so is the construction.
Panchayat approval is just getting a no objection from the local panchayat.Unless the panchayat has delegated powers from CMDA/DTCP to approve, any go-ahead from them is useless.
I am not too sure about your contention that if you get 1.5 FSI approval before ARZ rules apply, later you will be exempted.Even I have heard that if one had bought say 1 ground land in say palavakkam before 2006,they can get approval for building plan and the min 4800 sqft extent also doesnt apply.Maybe some one more experienced dealing with ECR can clarify.
On a general note, I request all to take time to look at the proposed land usage by year 2026 in the following links
CMDA Land Use Maps
http://www.cmdachennai.gov.in/Volume...4_MP_Maps1.pdf
http://www.cmdachennai.gov.in/Volume...4_MP_Maps2.pdf
http://www.cmdachennai.gov.in/Volume...4_MP_Maps3.pdf
You can see how CMDA has visualized to change the coastline usage thru the period 2006-2026.Now that wont be possible if they keep approving new building constructions.
As of now in my view, If the same rules continue to apply.ECR in long term will become like Santhome, Foreshore estate with only selective pockets like MRC nagar being called posh while the parallel road and locations like Sholinganallur along OMR will quickly develop into a well established self sustained posh locality like Mylapore,R.A.puram uniformly commanding more demand by residents and a more safer bet.
Having said that, I also feel ECR has great potential to be a huge draw for tourists to chennai in future.If proper efforts are taken to develop mahabalipuram like goa and have better facilities and entertainment hubs instead of just resorts we can see even faster development.Who knows later government may also allow skyscrapers on the shoreline like in Miami.