Respect Sir, Greetings !!!
Having learnt your situation meticulously, Kindly observe my recommendations :
1. Does this construction agreement have to be registered or is notarisation sufficient?
It is not mandatory for registration but do register it because there is highest possibility for misunderstanding between dialogues exchanged pertaining to financial and other terms. Secondly there is a common problem with these builders that they exercise immense delay. In both the situations if agreement is registered you will gain an additional weight in the court of law. Note that you take service of expert real estate lawyer.
2. If registration needs to be done, is that in addition to notarisation or can it be done on non-notarised document?
As mentioned above, registry will surely gain weight over notary. I highly recommend for registry.
3. What is the stamp paper value to be used and does a consideration value need to be added to it? How is consideration value calculated?
Rs. 100 Bond will go and registration fee can be found in registration office where your property is situated.
4. Do I have to pay any stamp duty if registering the construction agreement?
Absolutely Yes, there will be a stamp duty which will vary upon jurisdiction. The fee that you will pay is nothing when campare of additional head aces you suffer on an account of disputes hereinafter.
Because I have dealt with several cases pertaining to your situation. Golden advice. Go ahead with registry.
Hope every thing is absolutely clear. Still any issues consult me. Good Luck !!! Adv. Masood.