Original CasasCápsula conceptual illustration; not a specific model or shipment.
Two prefab homes can advertise the same price per square metre without offering the same indoor space. Before deciding which is cheaper, ask which area divides the price and what that price includes. Changing the denominator can change the apparent offer without changing the home.
This guide addresses a specific decision: placing two quotations on a comparable basis. Space available for living and the complete cost still require their own checks.
Exterior measurements are not the same as usable indoor floor space
Multiplying external length and width can describe a rectangular envelope. That calculation does not deduct enclosing walls, partitions or other elements. Nor does it establish whether a terrace or exterior space is included. An irregular plan or several modules requires an identified measurement boundary rather than an assumed rectangle.
As a Spanish valuation reference for specified financial purposes, Article 4 of Order ECO/805/2003 distinguishes usable and built areas. Its usable-area definition deducts specified internal elements and excludes exterior space. The consolidated text reviewed incorporates the 2025 amendment. This is not presented as a universal rule for every advertisement or country.
A worked example: the same price, two ratios
Imagine a fictional €60,000 quotation with 40 m² of declared exterior area and 32 m² of confirmed usable floor area. Dividing by 40 gives €1,500/m²; dividing by 32 gives €1,875 per usable m². The total remains €60,000. These numbers are an arithmetic illustration, not market pricing or a real quotation.
Neither ratio is necessarily misleading in itself. The problem arises when you compare them without knowing the basis. Ask every price-per-area statement to identify its denominator and maintain the same pricing scope.
The table to request from each supplier
| Field | The answer you need |
|---|---|
| Compared total price | Currency, taxes and included items |
| Exterior measurements | Dimensioned drawing identifying the configuration |
| Usable floor area | Figure, measurement method and associated drawing |
| Terraces and porches | Separate areas and inclusion in or exclusion from the price |
| Price per m² | Written denominator and matching scope for both quotes |
Retain the table with the drawing’s date and revision. If customisation changes partitions or wall thickness, confirm the areas applicable to the new order. Do not automatically carry an earlier specification’s measurements into a changed model.
Do not silently add terrace space to the interior
Spanish cadastral documentation uses its own built-area criteria, including rules for certain covered exterior spaces. A cadastral figure should not silently become “space to live in” in a manufacturing quotation. Ask for separate figures even where the supplier uses an aggregated commercial area.
A terrace can add value to your project. A useful comparison states its area, finish and cost separately, while checking that the interior also works when you cannot sit outside. This guide does not calculate the permitted development area of your plot.
Match the price inclusions too
Comparing the area ratio for a structural kit with that of an equipped home introduces another difference. Before calculating, record kitchen, bathroom, services, finishes, transport and site work according to each offer. “Not supplied” is more accurate than inserting a zero for an unknown cost.
If one proposal has more usable space but excludes work, keep both distinctions visible. Then decide which configuration meets your needs and which complete budget you can afford. Price per square metre should assist comparison alongside the drawing and deliverables list.
Continue comparing
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Sources and scope
- BOE: Orden ECO/805/2003, artículo 4, texto consolidado
- Dirección General del Catastro: guía de certificación descriptiva y gráfica
Primary sources reviewed on 23 October 2026. Comparison lists and examples are CasasCápsula editorial work. Commercial statements are attributed to their supplier. This article contains no sponsorship or affiliate links.