Regional and municipal surcharges¶

The engine computes addizionale regionale and addizionale comunale only for the jurisdictions the request names:

• regione: the ISO 3166-2:IT subdivision code of the region, IT- and two digits, for example IT-45 for Emilia-Romagna or IT-25 for Lombardia (source: ISO Online Browsing Platform). The regional surtax is set separately by the autonomous provinces, so use IT-BZ (Bolzano) or IT-TN (Trento); IT-32 (Trentino-Alto Adige) is rejected. The codes are listed in REGION_CODES (ccnl_engine.surtax.jurisdiction.types, internal).
• comune_belfiore: the codice catastale (Belfiore code) of the municipality, one upper-case letter and three digits, for example F257 for Modena.

A malformed code (ER, Lombardia, IT45, f257) is rejected with InvalidInputError. A well-formed code without a row in the tax year table is not an input error: see the decisions below. A code left None is an unknown residence, not a residence without surtax. The regional surtax is due to the region, and the municipal one to the municipality, of the domicilio fiscale on 1 January of the tax year (D.Lgs. 446/1997 art. 50 c. 5: "alla regione in cui il contribuente ha il domicilio fiscale alla data del 1° gennaio dell'anno cui si riferisce l'addizionale stessa"; D.Lgs. 360/1998 art. 1 c. 4: "L'addizionale è dovuta alla provincia e al comune nel quale il contribuente ha il domicilio fiscale alla data del 1° gennaio dell'anno cui si riferisce l'addizionale stessa"). Without the code the engine cannot tell which surtax, if any, is due: when the employer is a withholding agent, the run records a residence_unknown decision on that surtax (below) and has a requirement_unresolved blocker on it (see Fail-closed payability).

"""Regional and municipal surtax: withheld the year after the conguaglio."""

from dataclasses import replace
from datetime import date
from decimal import Decimal

from ccnl_engine import (
    EmployerProfile,
    Employment,
    Headcount,
    PayrollEngine,
    PayrollRun,
    PeriodFacts,
    PeriodInput,
)
from ccnl_engine.inputs import (
    FamilyComposition,
    InpsBaseYtd,
    OpeningBalances,
    SeniorityFact,
    SenioritySource,
    SurtaxComponent,
    SurtaxObligation,
)
from ccnl_engine.inputs import Permanent

engine = PayrollEngine.bundled()

# The 2025 conguaglio, run by the previous provider, determined the 2025
# regional surtax and municipal saldo and the 2026 municipal acconto.
# import_opening_balances is the one entry point for totals the engine did
# not compute.
opening = engine.import_opening_balances(
    OpeningBalances(
        tax_year=2026,
        surtax_obligations=(
            SurtaxObligation.open(
                SurtaxComponent.REGIONAL_BALANCE, 2025, "IT-45", Decimal("330.00")
            ),
            SurtaxObligation.open(
                SurtaxComponent.MUNICIPAL_BALANCE, 2025, "F257", Decimal("110.00")
            ),
            SurtaxObligation.open(
                SurtaxComponent.MUNICIPAL_ADVANCE, 2025, "F257", Decimal("45.00")
            ),
        ),
        # No other employment of the worker in 2026, no recovery running.
        inps_bases=(InpsBaseYtd(2026, Decimal(0), Decimal(0)),),
        recoveries=(),
    )
)

request = PeriodInput(
    run=PayrollRun.regular(year=2026, month=1),
    payment_date=date(2026, 1, 27),
    employment=Employment(
        ccnl_slug="metalmeccanico-federmeccanica.json",
        level_code="C3",
        seniority=SeniorityFact(36, date(2026, 1, 1), SenioritySource.PAYSLIP),
        contract_type=Permanent(),
    ),
    employer=EmployerProfile(headcount=Headcount(100)),
    facts=PeriodFacts(
        regione="IT-45",  # Emilia-Romagna
        comune_belfiore="F257",  # Modena
        family_composition=FamilyComposition(),  # no dependant
    ),
    opening_state=opening,
)
result = engine.calculate_period(request)

# January, the first run of 2026 after the import, withholds one installment
# of each 2025 saldo, at most 11 from January (D.Lgs. 446/1997 art. 50 c. 4,
# D.Lgs. 360/1998 art. 1 c. 5): 330.00 / 11 = 30.00 (3802) and
# 110.00 / 11 = 10.00 (3848).  The 2026 acconto starts in March (art. 1 c. 5).
for line in result.remittance_summary():
    if line.account == "surtax":
        print(f"  {line.remittance_code}: {line.amount}")

# The 2026 surtax is determined only by the conguaglio of 2026 and withheld
# in 2027: before it, each annual decision is determined_at_conguaglio.
for decision in result.decisions:
    annual = "component" not in decision.inputs
    if decision.capability.startswith("addizionale_") and annual:
        print(f"  {decision.capability}: {decision.reason_code}")

# What is left of the imported debts after January.
for obligation in result.closing_state.cash.obligations.surtax:
    print(f"  {obligation.component}: residual {obligation.plan.residual}")

# Whether the run can be paid is the assurance, not the surtax lines: list
# what blocks it, if anything.
print(f"Calculation: {result.assurance.calculation}, payable: {result.is_payable}")
for blocker in result.blockers:
    print(f"  {blocker.code}: {blocker.feature} {blocker.detail}")

# Without the residence the surtax of 2026 is undetermined, not zero: each
# missing code records residence_unknown and names the fact to supply.
unknown = engine.calculate_period(
    replace(request, facts=PeriodFacts(family_composition=FamilyComposition()))
)
for decision in unknown.decisions:
    if decision.reason_code == "residence_unknown":
        print(f"  {decision.capability}: supply {decision.inputs['fact']}")
print(f"Calculation: {unknown.assurance.calculation}")  # incomplete

When the surtax is withheld¶

The surtax of tax year N is determined by the conguaglio of N and withheld on the payslips of N+1:

Art. 50 c. 4 reads "trattenuto in un numero massimo di undici rate, a partire dal periodo di paga successivo a quello in cui le stesse sono effettuate e non oltre quello relativamente al quale le ritenute sono versate nel mese di dicembre"; art. 1 c. 5 applies the same words to the municipal saldo and sets the acconto "in un numero massimo di nove rate mensili, effettuate a partire dal mese di marzo". The withholding remitted in December is the one of the November pay period, so the engine, whose conguaglio is the last payslip of the year, withholds:

• one installment on each regular payslip of the window, the maximum number of equal installments rounded to the cent, the last one taking the residual; the November payslip takes whatever is left;
• nothing on the extra-month payslips (tredicesima, quattordicesima), the December payslip or an adjustment run;
• on the last run of the employment, every residual at once ("in caso di cessazione del rapporto l'importo è trattenuto in unica soluzione", art. 50 c. 4; "l'addizionale residua dovuta è prelevata in unica soluzione", art. 1 c. 5). That run is also the conguaglio of its year: the regional surtax and the municipal saldo of the year are withheld on it, and no acconto of the next year is determined (CU 2026 instructions, point 29 "non dovrà essere compilato" at the cessazione).

When the acconto withheld in the year exceeds the municipal surtax due, the conguaglio gives the excess back on a SURTAX_REFUNDS line (CU 2026 instructions, point 26: the acconto "effettivamente trattenuto, al netto, quindi, di quanto eventualmente restituito"). The CU text covers the cessazione; the engine applies the same refund at the ordinary conguaglio, where the saldo "determinato all'atto delle operazioni di conguaglio" (art. 1 c. 5) is negative.

A termination run after the last withholding slot is a second conguaglio of the year: it determines the surtax of the year again on the final income, drops what the first conguaglio deferred, and withholds the difference from what an earlier conguaglio of the year already withheld.

A debt keeps the region or municipality of the year that determined it: the installments of N+1 do not read the regione and comune_belfiore of the N+1 runs.

First year computed by the engine¶

The engine withholds in year N only the surtax an earlier conguaglio determined. For an employment it computes from January 2026 the 2025 saldi and the 2026 acconto come from the conguaglio of 2025, run by the previous provider or by the employer before the engine: import them with OpeningBalances.surtax_obligations and municipal_advance_withheld. Without them no surtax is withheld in 2026 and the whole 2026 surtax is deferred to 2027. The law still requires the 2025 amounts to be withheld in 2026 by the employer that certified them.

For a worker hired during N with no earlier employment at the same employer, no acconto of N is withheld (the acconto is determined by the conguaglio of the year before), the conguaglio of N determines the whole municipal surtax as saldo, and the acconto of N+1 is computed on the income this employer paid in N.

Surtax decisions¶

On every run of a withholding agent, each surtax records one CalculationDecision on the tax year in result.decisions, with capability addizionale_regionale or addizionale_comunale and no inputs["component"]. For a jurisdiction named in the request, on the conguaglio its amount is the annual surtax of the tax year on the annual taxable income; on any other run the surtax is not determined. Its inputs hold the code, the table row name, the tax year and the taxable income, and its rule and rule_version the bundled ruleset.

A table_unknown decision comes with a CalculationIssue coded regional_surtax_unknown or municipal_surtax_unknown, on every run, not only on the conguaglio. Nothing is determined for that surtax and the period result, hence the year result, is incomplete: it must not be paid as is. The same holds for a residence_unknown decision: the run is incomplete, with a calculation_issue and a capability_not_computed blocker besides the requirement_unresolved one. Nothing of the surtax of the tax year is determined, withheld, refunded or left to withhold, on the conguaglio too, and no annual table is loaded; the installments carried in, which keep the jurisdiction of the year that determined them, are withheld all the same. On the conguaglio the municipal acconto of the tax year is, as always, not posted as an installment: the municipal saldo would absorb it, and that saldo is the amount the run cannot determine. Such a conguaglio opens no obligation for the next year either, so its closing state is marked history_known=False: every later run, the first of the next tax year included, has a missing_fact opening_state blocker until the conguaglio is recomputed with the residence (see Opening state).

The conguaglio and each installment record one more decision per component, with the same capability and inputs["component"] (regional_balance, municipal_balance or municipal_advance), inputs["reference_year"] and inputs["jurisdiction"]:

The surtax is due only when the IRPEF net of its deductions is due (D.Lgs. 446/1997 art. 50 c. 2 for the regional, D.Lgs. 360/1998 art. 1 c. 4 for the municipal): a worker whose deductions absorb the whole gross IRPEF owes neither.

Foreign tax credit¶

Both articles take the IRPEF net of the deductions and of the credit for taxes paid abroad: art. 50 c. 2 D.Lgs. 446/1997 names the "crediti di cui agli articoli 14 e 15" of the TUIR in its former numbering (art. 15 is now art. 165), art. 1 c. 4 D.Lgs. 360/1998 the "credito di cui all'articolo 165". See Foreign tax credit at the conguaglio: when the credit brings the net IRPEF to zero, both surtax decisions are no_irpef_due.

Regional rates of 2026¶

regionale-2026.json is taken from the MEF Dipartimento delle Finanze pages of the addizionale regionale, one per region or autonomous province (addregirpef.php?reg=NN&anno=2026, retrieved on 27 September 2026). Each row records the URL of its page, the MEF publication date and a derived provenance; no row has been checked by a named reviewer yet. Where MEF lists two delibere for 2026 (Molise, Puglia) the later one, which raises the rates under art. 1 c. 174 L. 311/2004, is bundled.

The provisions that depend on income only are computed:

Regional deductions never create a credit: the surtax is floored at zero. Income limits are checked on the IRPEF taxable income of the employment, also where the regional law refers to reddito complessivo (Valle d'Aosta) or adds income taxed separately (Bolzano).

Provisions for dependents or disability are not computed: per-child detrazioni (Bolzano, Campania, Piemonte, Puglia, Sardegna, Trento) and reduced rates for a disabled taxpayer or family member (Marche, Veneto). The row states them in dependent_provisions. When the request declares a child or a disabled dependent in family_composition, the regional decision is provisional with the issue regional_surtax_dependent_provisions_not_applied; the surtax withheld may be too high. The engine has no input for the disability of the worker, so the Veneto rate for a disabled taxpayer without dependents is not flagged.

Municipal rates of the tax year¶

The acconto of N+1 uses the rate and threshold "nella misura vigente nell'anno precedente" (art. 1 c. 4), which at the conguaglio of N are those of N; the saldo of N uses them too. comunale-2026.json is generated by scripts/knowledge/build.py from the MEF elenco generale CSV of 2026, 2025 and 2024, downloaded on 7 October 2026 (their sha256 is in the file notes). A municipality that published an applicable 2026 delibera has its 2026 rates (final). One that has not yet (0* in the list, 4,322 of 7,897 at download, Milano, Roma, Napoli and Torino included) keeps the rates in force in 2025, which stay in force without a new delibera (art. 1 c. 169 L. 296/2006): the row has rates_year: 2025 and an assumed record, and its decision is provisional with the issue municipal_surtax_prior_year_rates until the table is rebuilt from a later list. A delibera the list marks inapplicable (adopted after the deadline, for example Bova in 2026 or Varco Sabino in 2025) is skipped the same way, so rates_year can be 2024. A delibera can still be published until 20 December 2026. Municipalities without a surtax have a zero rate row. A municipality created by a merger with no delibera of its own (Castegnero Nanto, M439) has no row: its code is unknown and the result is not payable.

Exemptions for a category of income only (FLAG_NUOVA 5 and 6 of the list, for example lavoro dipendente up to 12,000 euro) are kept as text in specific_exemptions and not computed; a row with them is provisional with the issue municipal_surtax_specific_exemptions_not_applied. The refresh procedure is in Data operations; a row the script cannot read stops the build.