What the texts say, and what they do not
Fees and the terms of the appointment are freely agreed. Ley 2/1974 on Professional Bodies (Colegios Profesionales) prohibits professional bodies from setting "guideline scales" of fees (art. 14; unofficial translation), the only exception being the guideline criteria they may draw up solely for the assessment of legal costs (fourth additional provision). The professional body's validation (visado colegial), for its part, "shall in no case cover the fees or the other contractual terms" (art. 13). Neither the professional body nor the visado therefore checks that the design fits the client's budget; the scope of the visado is explained in the article on professional body validation.
The Building Act (LOE) regulates the appointment, but not the cost. The designer drafts the design "on the commission of the promotor" (art. 10.1) and, during the works, the director de obra (works director) prepares "at the request of the promotor or with the promotor's agreement, any modifications to the design" (art. 12.3.d). The law does not give any party the obligation to hold the client's budget. The roles of each professional are described in the article on the architect and the arquitecto técnico.
The practical conclusion: the obligation to design within a figure arises from the contract. If the appointment does not include it, the client will find it hard to claim anything when the design estimate comes in above expectations. If it includes it precisely, the client has a basis for requiring the design to be revised.
What the courts show
The tendency of published decisions confirms that the contract and the evidence decide the matter. In STS 750/2012 of 12 December, as reported by vLex, the Supreme Court (Tribunal Supremo) rejected the architects' claim to fees calculated on an actual cost that had been increased by their own defective work. No fixed tolerance threshold for cost overruns that could apply in the absence of agreement has been found in Spanish case law, which reinforces the need to write one down. The analysis of this liability, and how it differs from the LOE guarantees, is developed in the article on cost overrun in the design and the architect's liability to the client.
The eight clauses that protect the budget
Eight clauses are enough to give the client's figure real effect. There is no official model; the exact wording is best reviewed with a legal adviser. The table summarises their content and their purpose.
| Clause | What to write | What it is for |
|---|---|---|
| 1. Target budget | amount and the month of the price base | fixing the figure against which the design is measured |
| 2. Scope of the figure | whether it is net works cost, contract price or total cost, and which items it includes | preventing each party from talking about a different figure |
| 3. Tolerance | permitted deviation, as an amount or a percentage, for an unchanged brief | separating normal imprecision from breach |
| 4. Cost deliverables by stage | preliminary design estimate, outline design valuation, quantities and itemised budget for the detailed design | controlling cost at each stage, not only at the end |
| 5. Duty to warn | written notice as soon as an estimate exceeds the target or the tolerance | deciding before drawing any further |
| 6. Costing of brief changes | every request from the client is costed in writing before being incorporated, and adjusts the target if accepted | distinguishing what the client asks for from what makes the design drift |
| 7. Design revision | if the budget exceeds target plus tolerance with no change of brief, the designer revises the design until it fits, with no additional fees | giving consequences to the breach |
| 8. Fee basis | fixed amount or percentage calculated on the target budget, not on the final cost | preventing the overrun from increasing the fees |
The scope of the figure is the clause that prevents the most disputes. A target expressed as total cost obliges the designer to reason through fees, taxes or utility connections as well, which the designer does not control; one expressed as net works cost is easier to verify, but leaves the client responsible for the rest. A reasonable option is to set the designer's target as net works cost and for the client to keep a separate total cost budget, built as explained in the article on moving from the works budget to the total development cost.
The cost deliverables must take a comparable form. Asking at each stage for the same structure of trade sections, the same price date and an indication of what has been added or removed since the previous stage makes it possible to read the trend at a glance. A global figure with no breakdown gives no way of knowing whether the increase comes from quantities, from rates or from the brief.
The fee basis breaks a vicious circle. If fees are calculated as a percentage of the works budget, every increase in the design also increases what its author is paid. Setting the fees as an amount, or calculating them on the target budget, aligns interests; fees are freely agreed and nothing prevents this being agreed.
Fictitious example: how the tolerance works
The following example is fictitious and its amounts are not market references. A client commissions a design with a target net works budget of 1,200,000 euros at January prices, and an agreed tolerance of 60,000 euros for an unchanged brief. The revision threshold is therefore 1,260,000 euros.
| Situation | Adjusted target | Threshold (target + tolerance) | Design estimate | Consequence |
|---|---|---|---|---|
| No change of brief | 1,200,000 € | 1,260,000 € | 1,310,000 € | exceeds the threshold by 50,000 €: design revision with no additional fees |
| With a brief change accepted in writing and costed at 70,000 € | 1,270,000 € | 1,330,000 € | 1,310,000 € | within the threshold: no revision required |
The difference between the two rows is a document. In the second, the client asked for a change, the designer costed it in writing and the client accepted it: the target was adjusted accordingly. Without that document, the same design estimate of 1,310,000 euros would open an argument over whether the increase came from the brief or from the design. The two figures must be compared at the same price date; if the design estimate was calculated with later prices, it is first brought back to January.
During the works: the appointment still matters
The site direction appointment can also include cost obligations. The LOE entrusts the director de la ejecución (site works director) with preparing the interim certificates and the final account (art. 13.2.e), and the director de obra with approving them (art. 12.3.e). Checking what has been built is not the same as forecasting the final cost. If the client wants a periodic forecast cost report, with approved and pending extra works, it has to be commissioned expressly, from the dirección facultativa (site direction team) or from another professional.
No professional should be able to commit expenditure on the client's behalf without written delegation. Modifications to the design during the works are made "at the request of the promotor or with the promotor's agreement" (LOE, art. 12.3.d). The appointment can specify that this agreement is given in writing, with an amount, before the work is carried out. In a fixed-price building contract, where article 1593 of the Civil Code allows price increases for changes authorised by the owner, that clause is what prevents a verbal instruction on site from becoming an extra work.
Before signing the appointment
Before signing, the client runs five quick checks. All of them can be read in the text of the appointment, not in what was said at meetings:
- that the target figure is stated with its price date and its scope;
- that the tolerance is expressed as an amount or a percentage, for an unchanged brief;
- that each stage has a cost deliverable with a comparable structure;
- that the consequence of exceeding the threshold is written down and does not depend on a later negotiation;
- that the fee basis does not grow with the overrun.
What is not written down is negotiated later, from a weaker position. Once the detailed design has been delivered and planning permission applied for, the client has little ability to require a revision the contract does not provide for: the time and money already invested work against the client.
This article reflects the state of the rules on the date of verification, 21 October 2026, and is intended as professional guidance. It does not replace legal advice or an examination of the specific case.