Extra works under a fixed-price contract: article 1593 of the Civil Code

The text addresses a specific case: the construction of a building or other works for a lump sum (ajuste alzado), "on the basis of a plan agreed with the owner of the land" (unofficial translation). In that case, the architect or contractor "may not claim an increase in price even if the cost of wages or materials has risen; but may do so when some change has been made to the plan that produces an increase in the works, provided the owner has given his authorisation".

The rule therefore has two halves. The first shields the price from changes in the contractor's costs: if labour or materials go up, the risk is the contractor's. The second opens the door to a price increase on two conditions: there must be a change to the plan that produces more work, and the owner must have authorised it.

Article 1593 does not require the authorisation to be in writing, nor the price of the change to be agreed before it is carried out. It is enough that the owner has authorised the change. Legal commentary points out that García Goyena's draft Civil Code of 1851 did require written authorisation and an agreed price, and that the Code as enacted dropped those formalities (A. E. Gudín Rodríguez-Magariños, "El ius variandi en las obras con ajuste alzado", Anuario Jurídico y Económico Escurialense, 2011).

The consequence for the budget is direct. A change mentioned at a site meeting, accepted verbally and carried out without a price, can end up in the final account as extra works. The text does not give the client the defence of "I never signed it"; it forces the client to argue whether or not it authorised the change, and what it is worth.

According to the commentary cited, the trend in published decisions of the Tribunal Supremo (the Supreme Court) accepts verbal or tacit authorisation, and even authorisation inferred from the owner knowing about the works and not objecting. The same study cites, among others, the judgments of 26 December 1979, 19 October 1995, 26 June 1998, 20 March 2001 and 27 May 2005, and notes that those rulings treat the article as an interpretative rule, not a mandatory one. Another analysis published on elderecho.com sums up that "the statutory rule does not require that consent to be express or in writing" and cites to that effect, among others, the judgment of 18 April 1995.

It is not a uniform line. The same study mentions a judgment of 30 January 2008 which, in a contract priced by unit of measurement, gave effect to the clause requiring the quantities and prices of any additional work to be approved beforehand in a written annex. What emerges from those decisions is that the outcome depends largely on the evidence: who asked for the change, who knew about it, what was said on site, what was paid without objection.

If the article is read as an interpretative rule, the practical effect is clear: the parties can agree otherwise. A contract requiring a written instruction and an accepted price before any change is carried out in principle replaces the Code's weak rule with a stricter rule chosen by the client, provided it is genuinely applied during the works.

The fixed price covers rises in the contractor's costs. The text rules out changes in wages or materials as grounds for an increase. If the contract includes a revision formula, that formula applies, and it is tracked on its own line of the dashboard (see price revision and the date of the budget).

The fixed price covers the agreed plan, and only that. The more complete the design at signature, the sharper its boundary. A design that leaves decisions for later (finishes, equipment, options) manufactures future extra works, legitimate if authorised. The differences between fixed price and unit rates are covered in the guide to quantities and budgets.

The fixed price does not cover changes requested by the client. Every change of brief during the works that adds work is an increase in the works that the contractor can charge for if the owner has authorised it. The Building Act (Ley de Ordenación de la Edificación, LOE) fits this logic: the director de obra (the works director) prepares any changes to the design "at the request of the promotor or with its agreement" (art. 12.3.d); the reach of that law on cost is analysed in what the LOE says, and does not say, about construction cost.

Since the law imposes no form, the procedure has to be written into the contract before work starts. Its effect on the budget comes down to four steps.

1. The request. Every change, whether it comes from the client, from the dirección facultativa (the site direction team) or from the contractor, is documented on a numbered change form describing the change and the items affected.

2. The valuation. The contractor submits a price, up or down, and its effect on the programme. If the change introduces units with no price in the contract, a new rate is set; the mechanics of new rates (precios contradictorios) are explained in the quantities guide. Omissions are recorded as rigorously as additions: an item removed and not deducted is a disguised extra.

3. The decision. The client authorises the change and accepts the price in the same document. The client signs, or someone holding a written and limited power to do so: the site direction team advises and controls, but the spending decision belongs to the client.

4. Execution, then the invoice. No changed work begins before signature. Where there is genuine urgency, an immediate authorisation by email, followed by the price within the period set by the contract, is worth more than a verbal agreement.

The register feeds the dashboard described in tracking the final cost during the works: signed forms go on the approved changes line, valued but unsigned ones on the changes under discussion line. The general mechanics of change orders (modificados de obra) are in the quantities guide.

With article 1593 read as in the judgments cited, the client's silence counts against it. Knowing about work and not objecting can be interpreted as authorisation. That is why the defence starts on site, not at the final account.

Three reflexes reduce the risk. Object in writing, and without delay, to any work that appears neither in the design nor on a signed change form. Review the minutes of site visits and record that any change discussed remains subject to written authorisation and price. Never pay for extra works "to keep things moving" without a document fixing the amount.

On the dashboard, disputed extra works stay recorded at their probable amount. Removing them because the contract seems to favour the client is betting on the outcome of litigation.

In public works, article 1593 is not the reference. Changes to a works contract are governed by the Public Sector Contracts Act (Ley de Contratos del Sector Público), which allows modifications only for reasons of public interest and within percentage limits on the initial price. They are analysed in how far a public works price can grow.

The client can withdraw from the works at its own will, even once they have started. Article 1594 of the Civil Code allows this provided the contractor is compensated for "all his expenses, work and the profit he could have obtained from it". It is a way out, not a saving: its cost is calculated as one more option in the decision, as explained in the note on what the client's withdrawal costs.