Sir,
According to mega exemption Notification No. 25/2012-ST dated 20.6.2012 as amended only the following services provided to Government are exempted from service tax-
Services provided to the Government, a local authority or a governmental authority by way of construction, erection, commissioning, installation, completion, fitting out, repair, maintenance, renovation, or alteration of -
(a) 38[omitted]
(b) a historical monument, archeological site or remains of national importance, archeological excavation, or antiquity specified under the Ancient Monuments and Archeological Sites and Remains Act, 1958 (24 of 1958);
(c) 38[omitted]
(d) canal, dam or other irrigation works;
(e) pipeline, conduit or plant for (i) water supply (ii) water treatment, or (iii) sewerage treatment or disposal; or
(f) 38[omitted]
Item (a), (c) and (f) shall be omitted vide Not. 06/2015 - Dated 1-3-2015 w.e.f. 1st of April, 2015,
“ (a) a civil structure or any other original works meant predominantly for use other than for commerce, industry, or any other business or profession;
(c) a structure meant predominantly for use as (i) an educational, (ii) a clinical, or (iii) an art or cultural establishment;
(f) a residential complex predominantly meant for self-use or the use of their employees or other persons specified in the Explanation 1 to clause 44 of section 65 B of the said Act;”
Therefore in my opinion you have to pay service tax in respect of the services provided by you to CPWD.