Accounts payable for construction companies

A construction SME with 12 simultaneous projects receives between 400 and 800 invoices per month. Subcontractors, materials, machinery rental, transport, workshops, engineering, waste management, utilities. Each invoice must be allocated to a specific project, reconciled against its PO or its certification, have the personal income tax withholding calculated where applicable, the reverse charge validated where relevant, the traceability preserved and only then written into the ERP. Doing it by hand is the scenario this guide describes first. Automating it is the second.

The sector also has an operational quirk no other vertical replicates with the same intensity: invoices do not arrive at month-end. They arrive continuously during project execution, during admin close-out and during the two following months for final adjustments, snags and final certifications. Without a system that allocates them correctly, they end up booked against the wrong project and the real cost of each project becomes distorted. And with it, the calculated profitability.

What makes construction billing special

Summarizing the sector as "lots of invoices" misses the point. What sets construction AP apart is the simultaneous combination of variables that appear in isolation in other sectors.

Subcontractors as the cost core

Between 40% and 70% of a construction project's cost in Spain is executed via subcontractors: structure, envelope, MEP, finishes, landscaping. Each subcontractor is a recurring vendor with its own contract, its own measurements and a monthly or bi-weekly certification calendar. The invoice is not issued against a closed PO; it is issued against the certified progress.

Progress certifications

A certification is the valued measurement of work executed during a period. The site manager signs it. The contractor submits it. The developer validates it. The invoice follows the certification with the previously agreed amounts, deducting the prior certification. Reconciling the invoice with the certification is a three-way matching where the "delivery note" is the signed record of work executed, not a goods delivery note.

15% personal income tax withholding on self-employed subcontractors

Spanish Law 36/2014 confirmed the 15% rate on the taxable base for self-employed workers. It applies to self-employed individual subcontractors providing professional services or business activities under direct estimation or modules as applicable. The payer withholds at invoice time, pays it through quarterly form 111 and reports it annually on form 190. The net amount paid to the subcontractor is base + VAT – withholding. Calculating it by hand for every invoice is a frequent source of error.

Reverse charge

Article 84.One.2.f of Spanish Law 37/1992 on VAT establishes the reverse charge for construction works executed between businesses, whether contractors or subcontractors, arising from contracts directly formalized between developer and contractor or between contractor and subcontractor. The subcontractor invoice carries no output VAT. The recipient self-assesses VAT on form 303 (output VAT line and input VAT line simultaneously). Intra-EU material purchases follow a similar self-assessment regime. The AP system must identify these operations automatically so the entry reaches the ERP correctly.

On-account advances

Before starting a project it is common to pay an advance to the subcontractor (typically 10%-20% of the contract). That advance is deducted proportionally from each subsequent certification. Manual tracking of advanced vs amortized balance is prone to errors and the subcontractor does not always remember it. The AP system has to keep the running advance balance automatically.

Deviations from site unknowns

A construction project is a unique work executed in a unique location. Unexpected geotechnical findings, regulatory changes, client modifications, material substitutions due to supply shortages. Deviations from the initial budget are not pathology, they are the norm. The difference between a profitable construction company and an unprofitable one is whether deviations are documented via change orders or quietly slip through invoices larger than the PO.

Milestone-based payments

Payment is usually tied to milestones: foundations complete, structure done, MEP wrapped, project handover. Invoices accumulate between milestones and payment is released when the milestone is validated. Without a system that maintains the invoiced vs paid balance per project, control happens manually in a spreadsheet that ages badly.

The 6 typical AP pain points in construction

Any admin or site manager in a construction company recognizes the following six as recurring. This section describes them with their regulatory or operational basis so the conversation with the tax advisor and with the ERP can be precise.

1. Subcontractors with 15% withholding

The self-employed individual subcontractor's invoice arrives with the 15% withholding applied on the taxable base. If it is wrong (incorrect amount, incorrect base, subject who should not be withheld), the payer is jointly liable to the Spanish tax authority for the correct payment. Law 36/2014 set the current rate. The AP system must verify the withholding calculation before proposing the journal entry. The typical manual error is calculating the withholding on base + VAT instead of on the taxable base.

More on this in the blog article about 15% withholding on subcontractors.

2. Progress certifications vs final billing

The subcontractor submits monthly certifications by progress. At project close there is a final certification that adjusts differences (real vs projected measurements, unexecuted items, additional items). The difference between the sum of partial certifications and the final certification can be positive or negative. Matching must contemplate this adjustment explicitly; you cannot reconcile "invoice against full contract" because billing is in chunks.

3. Reverse charge VAT

It applies in two scenarios: construction works executed between businesses under article 84.One.2.f Spanish VAT Law, and intra-EU material acquisitions. The invoice arrives with no output VAT. The system must identify the scenario, flag the operation as reverse charge in the accounting entry and record output and input VAT simultaneously so form 303 reconciles. Confusing a reverse-charge invoice with a no-VAT invoice by clerical error is one of the typical findings in a tax audit.

4. Site unknowns: invoices with deviations >10%

A construction company with budget discipline tolerates deviations up to 2-3% over the initial PO. Beyond that range, the invoice should be blocked until a documented change order exists, signed by the site manager and by management. In manual flows, invoices with 5-15% deviations get approved because "the difference is small" when looking at the total. Looking line by line, a 15% deviation on a line item that accounts for one third of the PO is a significant overpayment.

Pre-tax, line-by-line three-way matching as described in the three-way matching guide is the right tool to classify variance, not to suppress it.

5. Retention bonds

The contract with the subcontractor usually sets an additional retention bond (typically 5%) released after final project handover and the contractual warranty period (normally 12 months). That retention is not the withholding tax, it is contractual, and it accumulates in a liability account until released. Without a system tracking it per project and per subcontractor, releases get forgotten or executed twice.

6. 60-day payment terms vs real-world 90-120

Spanish Law 15/2010 amended by Law 18/2022 sets 60 days as the maximum payment term between businesses. The sector's historical practice has been 90-120 days, with known consequences for the network of SME subcontractors. Reducing the internal approval cycle (from the usual 6-14 manual days to hours) does not fix the payer's treasury decision, but it removes the "not approved yet" excuse and leaves the payment calendar to a conscious decision, not the admin bottleneck.

How to automate subcontractor reconciliation

The concrete workflow for a subcontractor invoice on an active project, described step by step. It is the most complex case in the sector: if the system can solve it, the rest of the flow (materials, rentals, utilities) is trivial by comparison.

1. Ingestion from the email inbox

OAuth connection to the admin or project inbox. The system reads incoming emails, identifies those containing invoices (a PDF attachment or an image such as a phone photo or scan) and extracts the files. Never modifies or deletes. Ingestion is continuous: an invoice sent by a subcontractor at 6:00 PM is available in the system within minutes.

2. Supplier and project identification

The system recognizes the subcontractor's tax ID and links it to the active contract. If the subcontractor works on several simultaneous projects, the system uses the invoice description, the project code if present, or asks in a structured way before continuing. Never assumes.

3. Line extraction and withholding validation

OCR and IDP extract the lines: description, unit of measure, quantity, unit price, base, VAT applied (or absent if reverse charge), withholding applied. The system verifies that the withholding is calculated on the taxable base and at the correct rate (15% for self-employed under general regime, other rates depending on the case). If the calculation does not match, the invoice goes to exception queue.

4. Matching against contract + certification + material delivery note

The system locates the subcontractor's contract (agreed measurements and amounts), the period's progress certification (signed by the site manager with the executed percentage) and any material delivery note provided by the subcontractor if the contract requires it. Reconciles line by line, pre-tax: certified units vs invoiced units, contractual unit price vs invoiced unit price.

5. Withholding and net-payable calculation

The system automatically calculates the net payable as base + VAT (if applicable) – withholding – advance amortization if any – contractual retention if applicable. The result is shown to the approver with full detail, not as a single aggregate total.

6. Site manager approval

The site manager receives in their inbox the invoice view, the signed certification and the reconciled lines. One button approves the technical side (yes, the work has been executed according to the certification). If there is variance, they leave a tagged comment (new unforeseen item, measurement error, material substitution). The comment goes to the audit trail.

7. Admin approval

Admin receives the technically validated invoice. Only verifies the administrative aspects: amount within project budget, correct withholding, reverse charge applied where applicable, subcontractor IBAN matching history (classic anti-fraud control). One click.

8. Accounting in the ERP

The approved invoice is exported to the ERP with the correct project allocation, the corresponding accounting accounts (subcontracting, input VAT, withholdings payable) and the calculated due date. The journal entry arrives balanced and needs no manual intervention by the accountant.

The amount-based routing component is described in the guide about automating supplier invoice approval.

Three-way matching applied to construction

Traditional three-way matching reconciles invoice vs PO vs goods receipt. In construction the three documents change nature depending on the operation. The table below summarizes the four variants that appear on any project.

Operation	PO (what was agreed)	Delivery note (what was received)	Invoice
Material to site	PO to the supplier with quantities and prices	Delivery note signed by the foreman	Material supplier invoice
Subcontracting by certification	Subcontracting contract with measurements	Progress certification signed by the site manager	Subcontractor invoice
Machinery rental	Rental contract with rate and term	Signed timesheet of usage hours	Rental company invoice
Professional services (engineering)	Professional engagement with fees	Signed deliverable (drawings, reports)	Professional invoice

Variance formulas

The system calculates two variances per line, always on pre-tax unit prices:

• price_variance = (invoice_unit_price − po_unit_price) × invoice_qty
• qty_variance = (invoice_qty − po_qty) × po_unit_price

Default tolerances are 1.50 EUR absolute or 2% relative, OR combinator (the line is variance if either of the two dimensions is exceeded). The comparison is strict: a value exactly equal to the threshold is within tolerance. Any line out of tolerance generates a variance classified by cause: price, quantity, new line item, VAT, withholding.

Why pre-tax

Comparing totals with tax included mixes the variance of the real operation with the variance of the tax rate. The same material can carry 21% or 10% VAT depending on the type of project (rehabilitation of primary residence vs new build); the reverse charge changes the presence or absence of VAT depending on the subject. If the comparison is on totals, the reading is confusing. Pre-tax, line by line, the reading is precise.

Extended argument in matching by totals is a trap and in the supplier invoice control guide.

Special case: cumulative certifications

The month 3 progress certification does not invoice only month 3's work; it invoices the cumulative executed as of month 3 minus what was already invoiced in prior certifications. Matching must work on the delta, not on the cumulative. It is a simple arithmetic detail but conceptually important: the system keeps a running account per subcontractor and per project, updated with each certification.

Tax compliance: Verifactu and construction

Construction companies are affected by Verifactu both on the issuing side (when invoicing the developer or final client) and on the receiving side (when receiving invoices from suppliers under Verifactu). Royal Decree-Law 15/2025 set the staggered rollout calendar. The operational details on the receiving side are developed in Verifactu for paying companies.

On the issuing side

The construction company that bills the developer (certifications, final handover, final settlement) must issue under an invoicing IT system compliant with Royal Decree 1007/2023 if volume and activity require it. The issued invoice includes the QR code and chained hash.

On the receiving side

The construction company that receives invoices from subcontractors and suppliers under Verifactu must be able to:

• Verify the QR against the Spanish tax authority (optional but recommended external validation).
• Preserve the chained hash as auditable evidence.
• Record receipt date and time for payment-term computation.
• Preserve the document during the tax statute of limitations (4 years generally, 6 years for accounting purposes).

ininvoice records the receipt date and time on ingestion, preserves the original invoice and keeps an auditable trail exportable for audit. Verifying the QR against the Spanish tax authority is an optional external step and stays outside ininvoice.

Real case: developer with 12 simultaneous projects, 600 invoices/month

Numerical example taken from the typical Spanish SME property developer profile. Representative data, no attribution to a specific company.

Profile

• Developer with 12 active projects across 4 provinces.
• 600 invoices received per month on average. Peaks at 850 in quarter-end.
• 45 regular subcontractors, 70 recurring material suppliers, 30 occasional ones.
• Admin team: 3 full-time people.
• ERP: Sage 200 Advanced Construction.

Previous manual flow

Of the 600 monthly invoices, 480 reconcile without incidents and 120 present some kind of deviation or anomaly. Average time per clean invoice is 9 minutes (open, read, find the PO or certification, compare, key into ERP, file). Those with deviations consume between 25 and 60 minutes each. Monthly total: 480 × 9 min + 120 × 40 min = 4,320 + 4,800 = 9,120 minutes, i.e. 152 hours. Equivalent to 95% of a full-time role just approving invoices.

First-month findings with ininvoice

• 23 invoices with unit-price deviation above 5% versus PO. Aggregate variance amount: 4,700 EUR.
• 4 duplicate invoices (same subcontractor, same certification, two separate sends). Amount: 11,300 EUR.
• 11 invoices with miscomputed withholding (calculated on base + VAT instead of on the taxable base). Aggregate difference: 380 EUR.
• 2 invoices with output VAT wrongly charged on a reverse-charge operation. Obvious tax risk.
• 9 invoices booked to the wrong project in the previous manual flow, caught by cross-checking line descriptions.

Time and savings

After stabilization (month 2 onward), the time spent by the admin team on invoice approval drops from 152 hours/month to 28 hours/month (exception handling only). The freed equivalent is reassigned to project control, bank reconciliation and month-end close. The process-cost saving, valued at internal rate, is described in the ROI section below.

Integration with construction ERPs

ininvoice works with any ERP that imports CSV or exposes an API. In the construction sector the usual destinations are the following.

Sage 200 Advanced Construction

The most widely deployed ERP in mid-size Spanish construction. Supports projects as an allocation dimension, measurements, certifications, subcontracting and machinery. ininvoice exports validated invoices with the correct project, accounting account, VAT and withholding. Sage flow details in Sage + supplier invoice control.

A3 ERP with sector modules

Common in accounting firms that run accounting for SME construction companies and developers. ininvoice exports balanced journal entries directly to the A3 model. Details in A3 + supplier invoice control.

Sector-specific Holded

For SME developers with light structures and smaller projects (up to 5-8 simultaneous). Lets you use labels and projects as a proxy for projects. ininvoice writes to Holded via API.

Presto and Arquímedes

Classic measurement and budgeting tools in the sector. They are not accounting ERPs, but they are the source of measurements and certifications. The natural integration is to import the initial budget as the matching base and export the billing to the accounting ERP.

ininvoice does not replace the ERP. It sits between the email inbox and the ERP as a layer of ingestion, OCR, matching and approval.

ROI: how much you save by automating construction AP

The figures in this section use the sector data from Ardent Partners for the average cost of processing an invoice manually: between 9 and 16 EUR per document in mid-size companies, vs less than 3 EUR per document in best-in-class companies (State of ePayables 2024).

Construction company with 600 invoices/month

Item	Manual (range)	Automated (ininvoice)
Cost per invoice	9 – 16 EUR	< 3 EUR
Monthly cost (600 invoices)	5,400 – 9,600 EUR	1,800 EUR
Monthly saving	3,600 – 7,800 EUR
Annual saving	43,200 – 93,600 EUR

The ininvoice plan is at 49 EUR/month up to 200 documents/month; construction companies with larger volumes take the next tier, with no implementation cost and no commitment. Payback on the tool cost lands under one month.

What does not appear in the table

The tabulated saving measures process cost only. Three additional items have direct monetary impact but are not included because they vary too much across companies:

• Overpayments avoided through line-by-line matching. In the example above, 4,700 EUR of aggregate price variance in a single month. Annualized, around 50,000 EUR for a 12-project construction company.
• Duplicates detected. 11,300 EUR in one month in the example. Duplication rate in construction typically 1.5%-3% of received invoices, driven by the practice of resending certifications.
• Financial cost of admin delay. Cutting the internal approval cycle from 6-14 days to hours removes the delay excuse and improves the relationship with SME subcontractors, who are the most sensitive to payment terms.

The extended and customizable calculation is available in the ROI calculator.

Frequently asked questions

Why is construction billing more complex than distribution or retail?

Because it combines four variables that retail does not have: self-employed subcontractors with 15% personal income tax withholding (Spanish Law 36/2014), progress-based work certifications that are billed partially, reverse charge VAT in real-estate operations and intra-EU material purchases, and recurring deviations versus the initial purchase order driven by site unknowns. A construction company with 600 monthly invoices manages 12 to 30 simultaneous projects, each with its own budget, subcontractors and material line items.

How do you apply three-way matching when the invoice is from a subcontractor instead of a material supplier?

Three-way matching is adapted: the PO is the subcontracting contract with its measurements and amounts, the delivery note is the work certification signed by the site manager (a record of work executed) and the invoice is the subcontractor's. The system reconciles line by line: certified units vs invoiced units, contractual unit price vs invoiced unit price, and automatically calculates the 15% withholding before proposing payment.

What withholding rate applies to construction subcontractors and where is it regulated?

The general rate is 15% on the taxable base for self-employed workers providing professional services or business activities. Construction additionally has the reverse charge regime governed by article 84.One.2.f of Spanish Law 37/1992 on VAT for construction works executed between businesses. Law 36/2014 confirmed the 15% rate after several increases and decreases in previous years. Always check with your tax advisor before configuring automated rules.

What is the reverse charge in construction and when does it apply?

The reverse charge shifts the obligation to declare and pay VAT onto the recipient. In construction it applies in two cases: works executed between businesses arising from direct contracts between developer and contractor or between contractor and subcontractor (art. 84.One.2.f Spanish VAT Law), and intra-EU acquisitions of materials. The subcontractor invoice carries no output VAT and the developer self-assesses VAT on form 303.

How do you handle an invoice with a deviation versus the PO due to site unknowns?

The system detects the variance line by line, classifies it (price, quantity, new line item) and routes it to the site manager for validation. If the deviation is justified (geotechnical finding, regulatory change, client modification) a change-order PO is generated that updates the project budget. The invoice is only approved when the modification is documented; it is not approved "because the difference is small" the way it happens in manual flows.

Does Verifactu affect construction companies?

Yes, both as issuer and as receiver. As a payer, from the calendar set in Royal Decree-Law 15/2025 invoices from suppliers under Verifactu arrive with QR and chained hash verifiable against the Spanish tax authority. The receiver's obligation is to preserve the traceability of the received invoice. ininvoice records the receipt date and time, preserves the original invoice and keeps an auditable trail; verifying the QR against the tax authority stays an optional external step. Details in Verifactu for paying companies.

Which construction ERPs integrate with ininvoice?

ininvoice works with any ERP that imports CSV or exposes an API. In construction the usual destinations are Sage 200 Advanced Construction, A3 ERP with sector modules, Holded with project templates, and Presto/Arquímedes for measurements. The AP automation layer sits between the email inbox and the ERP: ingestion, OCR, three-way matching, withholdings, and then writes the accounting entry with the correct project allocation.

How much can you save by automating AP at a construction company with 600 invoices per month?

Ardent Partners puts the average cost of manually processing an invoice between 9 and 16 EUR per document. For 600 monthly invoices that is between 5,400 and 9,600 EUR per month in process cost. Best-in-class companies stay below 3 EUR per invoice, i.e. 1,800 EUR/month for the same volume. The monthly saving lands in the 3,600 to 7,800 EUR range, not counting undetected overpayments from site deviations or the financial cost of late payments.

This page describes the operational behavior of the product as of the update date. The savings figures come from Ardent Partners State of ePayables 2024 and from representative sector projects. Regulatory references (Spanish Law 36/2014, Law 37/1992, Royal Decree 1007/2023, Royal Decree-Law 15/2025) should be verified with a tax advisor before configuring automated rules in production.