It is presumably understood from the query is that subject supply under consideration here is with respect of new flat under construction by the builder and subject supply is with relating to electrical wiring fitting with switch-board etc., as generally provided to the builder while selling flat to the buyer.
In my view, same is 'Works Contract Services' which is defined u/s 2 (119) of the CGST Act, 2017 as under:
“works contract” means a contract for building, construction, fabrication, completion, erection, installation, fitting out, improvement, modification, repair, maintenance, renovation, alteration or commissioning of any immovable property wherein transfer of property in goods (whether as goods or in some other form) is involved in the execution of such contract"
Relevant explanatory notes to scheme of classification of services under GST issued by GOI:
9954: Construction services
995461: Electrical installation services including electrical wiring & fitting services, fire alarm installation services, burglar alarm installation services
This service code includes
i. special trade installation services involving the installation of basic electrical wiring circuits or fittings in buildings and other construction projects;
ii. electrical wiring and fitting services for emergency power supply systems;
iii. electrical services arising from the installation of appliances;
iv. installation services of meters; installation services of fire alarm systems;
v. installation services of burglar alarm systems at the construction site;
vi. installation services of all types of residential antenna including satellite antenna;
vii. installation services of cable television lines within a building; installation services of lightning conductors;
viii. electrical installation services of telecommunications equipment;
ix. electrical installation services of illumination and signalling systems for roads, railways, airports, harbours and similar premises;
x. installation services of heavy electrical equipment;
xi. installation services of telecommunications wiring, including of fibre optic cables;
xii. other electrical installation services n.e.c.
These are ex facie views of mine and the same should not be construed as professional advice / suggestion.