GST on works contracts explained for contractors — the 18% rate, ITC rules under Section 17(5), SAC 9954 on RA bills, GST TDS, and common GSTR mismatches.

Few subjects burn more contractor working capital than GST handled casually. A works contract sits in an odd corner of the GST law — a composite supply of goods and services that the statute deliberately treats as a service — and the practical consequences reach into every RA bill, every advance, every subcontractor payment, and every monthly return. This guide covers the essentials a contractor's billing and accounts team must get right: the rate structure, input tax credit rules, SAC codes on RA bills, GST TDS on government work, and the return mismatches that generate notices.

Works Contracts Under GST: The Basics

A works contract under Section 2(119) of the CGST Act is a contract for building, construction, fabrication, erection, installation, repair, or commissioning of any immovable property, where transfer of property in goods is involved. Concrete, steel, and labour go in; a building, road, or bridge comes out.

Two foundational points:

It is treated as a supply of services under Schedule II, regardless of the goods content. One contract, one SAC code, one rate — you do not split the invoice into a goods portion and a services portion.
The standard rate is 18 percent for most works contract services under SAC 9954. A handful of concessional entries survive — certain earthworks-dominant government contracts and specified affordable-housing constructions — but the safe default for commercial, industrial, institutional, and most government civil work today is 18 percent.

Contractors who lived through the earlier regime remember 12 percent on government works contracts. That ended in stages through 2022, when the concessional 12 percent entries for works supplied to governments and local authorities were moved to 18 percent. Old habits die hard: bills still surface carrying 12 percent on contracts that no longer qualify, and the shortfall — plus interest at 18 percent per annum — lands on the contractor.

Input Tax Credit: What You Can and Cannot Claim

ITC is where works contracts get genuinely confusing, because the answer depends on who you are in the chain.

When ITC Is Available

A works contractor providing works contract services can claim ITC on inputs — cement, steel, aggregates, diesel-excluded consumables, shuttering material, hired machinery, subcontractor invoices — because those inputs go into an outward taxable supply of works contract services. This is the normal case for a contractor building for a client.
A subcontractor's works contract invoice is creditable to the main contractor for the same reason: works contract service received for providing an onward works contract service is expressly outside the Section 17(5) block.

When ITC Is Blocked

Section 17(5)(c) and (d) block ITC on works contract services and on goods or services used for construction of immovable property on one's own account — even when used in the course of business. Practical translations:

A developer constructing a building to sell after completion (no GST on the sale) cannot claim ITC on construction inputs.
A factory owner building a new plant cannot claim ITC on the civil works contractor's invoice, though credit on plant and machinery — a distinct category — survives.
A contractor building its own office or workshop cannot claim ITC on those inputs.

The line to hold: ITC flows when construction is your outward supply; it stops when the construction is for your own books. Diesel deserves a special mention — it is outside GST entirely, so no ITC ever, which is one more reason fuel leakage hurts twice.

Also remember the working-capital mechanics: ITC is available only when the supplier files, the invoice appears in your GSTR-2B, and payment is made within 180 days. A supplier who takes your money and skips filing costs you the credit plus interest.

SAC Codes on RA Bills

Every tax invoice raised against an RA bill must carry the right Services Accounting Code. Works contract services live under heading 9954, with six-digit codes by construction type — general construction of buildings, roads and railways, utility projects, and so on. Practical rules:

Turnover above ₹5 crore requires six-digit SAC on invoices; smaller contractors may use four digits, but six is good practice on works contracts because departments increasingly validate them.
Use one SAC per contract consistently across every RA bill; switching codes mid-contract invites scrutiny.
The SAC on the invoice must match what your GSTR-1 reports — mismatches here are among the easiest automated flags for the department to raise.

Because the tax invoice must be raised on the certified value within 30 days of certification, the billing module that generates the RA abstract should generate the GST invoice too — one source, no retyping. This is exactly how BOQ & billing treats the RA-to-invoice flow.

GST TDS on Government Contracts

On contracts with government departments, local authorities, and notified PSUs, the client deducts GST TDS at 2 percent (1 percent CGST plus 1 percent SGST, or 2 percent IGST) on payments where the contract value exceeds ₹2.5 lakh. The deducted amount lands in your electronic cash ledger once the deductor files GSTR-7, and you claim it there — it is not a cost, but it is cash flow timing you must track. A contractor running multiple government contracts should reconcile GST TDS credits monthly; unclaimed TDS sitting in the cash ledger while you pay tax through the bank is pure negligence.

Alongside GST TDS, remember the parallel deductions on the same bill — income-tax TDS under 194C and labour cess at 1 percent — each with its own certificate and its own reconciliation. Three deduction streams, three chances for the client's computation and yours to diverge.

The GSTR Mismatches That Trigger Notices

Most GST notices to contractors are not about fraud; they are about reconciliation gaps between returns. The recurring ones:

GSTR-1 versus GSTR-3B. Invoices reported in GSTR-1 but tax short-paid in 3B — often because a credit note against a certified-lower RA bill was netted informally instead of being documented.
GSTR-3B versus GSTR-2B on ITC. Credit claimed on invoices the supplier never filed. With sequential filing and auto-populated 2B, excess claims surface within months, with interest at 18 percent.
Advances not taxed. Mobilisation advance received but GST not paid at receipt — a classic works contract slip, because the RA billing team thinks of the advance as a loan, not a supply.
Books versus returns at year-end. The GSTR-9 annual return forcing recognition of unbilled certified work, retention-value differences, and free-issue material adjustments that monthly returns papered over.
E-way bill and invoice mismatches on material moved between your own sites versus material billed to clients.

Every one of these traces to the same root cause: billing data living in one spreadsheet, measurement data in another, and the accountant reconstructing the month after it ended.

Getting the Data Right at Source

The contractors who stopped fearing GST notices did not hire bigger consultants — they fixed the pipeline. When certified RA values, credit notes, advances, and deductions flow from the billing system into the accounts view automatically, the monthly returns become a report instead of an investigation. That is the design behind financial control working off the same records as billing: one version of every number, from measurement book to GSTR-1.

GST on works contracts rewards exactly one behaviour: contemporaneous, structured records. The rate is what it is; the leakage is optional.

Book a ProjectSathi demo at /contact to see RA billing, GST invoicing, and deduction tracking run off a single set of site records.