Only after layout plan is sanctioned should registration of flat be done in sub registrar office 

Without even getting an approval for construction of the floor in which yo have been proposed to be allotted with a flat, there is no purpose to be solved by getting a property tht is not existing.

It is not even advisable to pay any more amount towards advance even before the plan for 18th floor is approved and without any commencement of the construction.

If you have reason to believe that approval for 18th floor is likely to be granted then you can proceed to register the deed of conveyance.

Else wait till it is done.

Yes it can be registered 

It is better to await the building-plan approval from the authorities concerned for construction of the 18th floor. In case approval is not granted for some reason, you will have to be at the builder's mercy for refund of the sale consideration paid, if the flat is registered pending approval now.

there are 2 risks

one is that the flat being sold to you is not in physical existence

second is that it does not even reflect in the approved plans

usually people go for a registered agreement for sale when the flat being sold to them is clearly shown on the sanctioned plans and there is only 1 risk then - that of the builder defaulting in constructing the promised flat within the agreed time

so in your case you will be taking a twin risk

also for registering the agreement you will have to shell out stamp duty and registration charges

so what in case the builder does not submit further plans for approving construction above 16 floors ?

so i think your apprehension is valid

ask the builder to give you an indemnity bond to protect you against the above said scenario. though an indemnity bond is merely a piece of paper but in case of default you can invoke it against the builder and sue him for recovering your invested money with damages

in case you have availed a bank loan, the bank in the first place will not sanction the loan if the flat under sale is not reflecting on the sanctioned building plans

- As per the RERA, builders are required to adhere to the sanctioned maps and layout plans approved by the competent authorities prior to starting a real estate project.

- Since , the builder has not granted layout plan for the said extended floors then it may create trouble for you if said approval is rejected due to any unforeseen reason.

- However, for the execution of the sale deed , the layout plan is not mandatory . 

I understand you're considering registering a flat on the 18th floor of an under-construction building in Mumbai. Here's some information to help you decide, but it's important to understand this isn't legal advice.

Registering Flat with Pending Layout Plan:

Potential Risk: Registering a flat where the layout plan for your specific floor (18th) is unavailable might be risky.Approval Uncertainties: Without the approved layout plan, there's no guarantee the 18th floor will be built as envisioned, impacting the flat's size, location, or configuration.

Key Points to Consider:

Developer's Assurance: While the developer might claim sufficient FSI (Floor Space Index) is available, rely only on official approvals, not verbal assurances.Sub-Registrar's Role: The sub-registrar might register the flat despite the missing layout plan, but it doesn't guarantee future approvals or the final configuration of your flat.

Recommendations:

Delay Registration: Consider waiting until the layout plan for the 18th floor is approved by the authorities. This provides clarity on the flat's specifics before registration.Consult Lawyer: A lawyer specializing in property law can review the agreement and advise on the risks and benefits of registering before the layout plan is finalized. They can also help negotiate terms that protect your interests in case of layout changes.

Additional Considerations:

Financial Implications: If the layout changes significantly after registration, you might have limited recourse, especially if the final flat size is smaller than what you envisioned.Time Factor: While waiting for the layout plan might delay registration, it could save you from potential issues later.