Yes if they pay premium they can get it 

per sanctioned modification to the Development Control Regulation for Gr. Mumbai-1991 vide Govt. Notification dt. 6th January 2012, Fungible Compensatory additional F.S.I. henceforth will be permitted over and above present admissible F.S.I. for all type of developments in the Greater Mumbai Limits with the special permission of the Municipal Commissioner subject to the terms and conditions mentioned therein against the payment of premiums. The sailent features of the said permissible incentive Fungible compensatory F.S.I. related to user/zone are described as under:-

a) Extent of Permissible Fungible Compensatory F.S.I.:-
Zone & Users Fungible F.S.I. permissible
i) Residential Development : 35% overadmissible F.S.I.

Government Notification dated 6th January 2012 and in particular amendment to D.C. Regulation 33(5) provides that Municipal Commissioner is empowered to permit fungible compensatory FSI, notwithstanding anything contained in the D.C. Regulations 32, 33 & 34 at a particular percentage by charging premium subject to the proviso that redevelopment under D. C. Regulations 33(5) and redevelopment proposal of existing buildings in suburbs and extended suburbs by availing TDR, the fungible bdp wp-1699.16(j).doc compensatory FSI admissible on FSI consumed in existing structure shall be granted without charging premium.

In case a developer constructs anything beyond the permissible FSI limit and increases the built-up area, then the extended development over and above the permissible limit is called the Fungible FSI. Builders have to purchase the space from the city authorities to carry out the development.

As per the norm, the fungible FSI should not exceed over 35 percent of the floor area in residential properties and 20 percent of the floor area in industrial and commercial developments.

The additional space purchased from the authorities is referred to as fungible FSI. It is also referred to as Premium FSI.

The FSI and the fungible FSI in an area vary as per the type of the building one plans to construct. To know about the prevalent FSI and the fungible FSI in the preferred area, one needs to visit the official website of the State government since it would aid in better planning and execution.

 residential properties attract 60 percent of the RRR as fungible FSI fee. Similarly, the fees for industrial and commercial developments are 80 and 100 percent of RRR, respectively.

Under Rule 65 of DC-34, “Fungible Compensatory Area” means any built-up area permitted over and
above the admissible FSI by a special permission of the Commissioner in
accordance with the Regulation No. 31(3). 

As per rule (EP-15) Statement of area of the holding by triangulation method from a licensed
surveyor or architect with an affidavit from the owner certifying the boundaries of the plot and area n the form prescribed by the Commissioner; 

D.C.REGULATIONS 33 :-
▶ 33(5):- Development or Redevelopment for housing schemes of MHADA;

Minimum entitlement of carpet area to existing tenant is existing carpet area plus 35% but in any case
minimum carpet area is 300.00 sq.ft. i.e. when plot area is upto 4000 sq.mt.

The plot under redevelopment means the land demarcated by MHADA for redevelopment, provided that the maximum entitlement  of rehabilitation area shall be in no case exceed the maximum limit

Regarding what?

that's incorrect

the developer can give bigger rehab flats to the tenants by availing fungible FSI from the BMC which is capped at 35% of the admissible FSI 

So for example if the total construction to be made as per available FSI is 10,000 sq.ft. then the developer can avail of extra 35% fungible FSI i.e. 3500 sq.ft.for residential/industrial/commercial development by paying a premium of 50% (of the premium mentioned in the Annual statement of rates or ready reckoner) for residential development and 60% for industrial and commercial development

so the developer can get extra 35% fungible FSI by paying a premium for construction of flats to be sold in open market

whereas for constructing rehab flats (for allotment to tenants), if he avails fungible FSI, then he is not required to pay any premium. However he can get at max 35% extra area and accordingly he can provide bigger rehab flats to the tenants 

so whatever FSI is needed to construct the rehab flats for tenants, the developer can apply for upto 35% extra FSI of the FSI needed to construct rehab flats, from the BMC, and give rehab flats of bigger area to the tenants