The Spanish term forjado sanitario usually describes a raised ground-floor arrangement with a space beneath it. Do not assume it means the accessible crawl space you know from another country. Some solutions have a low formed cavity that cannot be entered once the floor is complete.
Before requesting a price, establish what that space is supposed to achieve. Separating the occupied floor from the ground, accommodating planned services and supporting moisture-control measures are different functions. Each needs to appear in the design and budget.
Start with the section through the house
Read the section from the ground upwards: prepared support, foundations or other designed load paths, floor structure, insulation, service layers and finished surface. The void sits within that arrangement. It does not independently explain how the house transfers loads to the ground.
There are also different ways of forming it. Daliform’s Iglu documentation describes modular permanent formwork used to create cavities beneath a concrete floor. The plastic components form the space during construction; buying those components is not the same as buying the completed structural floor.
A suspended arrangement with sufficient designed access is a different proposition from a shallow network of cavities. If future access to services matters to you, specify that requirement at the briefing stage. A photograph of an empty void does not establish maintainability.
The broader foundation guide explains the role of ground support. Keep that scope separate enough to see whether two contractors are actually pricing the same building.
Ground information can change the preferred option
Your geotechnical investigation informs the structural solution. Ground movement, support conditions and water cannot be assessed from the choice of floor product alone. A formed cavity is not a universal answer to difficult soil.
Finished levels matter too. On a sloping plot, the floor may sit close to the ground on one side and much higher on the other. That changes edges, retaining conditions and access, so an estimate based on a flat diagram can miss significant work.
Think through the external ground after landscaping, not only the excavated site. Paths and planting can change the relation between ventilation openings and their surroundings. The finished arrangement should preserve the function shown on the drawings.
Where earthworks and excavation are separate contracts, establish their handover condition. The floor installer needs a defined starting point; otherwise an attractive unit rate may depend on preparation nobody has included.
Ventilation needs connected spaces and a destination
An air opening serves little purpose if it connects to an isolated pocket. The useful question is how air enters, reaches the intended spaces and leaves. Internal divisions and structural elements can interrupt that route, so the complete layout matters.
Spain’s CTE DB-HS addresses moisture protection and radon through different sections. A ventilated cavity can form part of a specified strategy, but the presence of a void does not by itself establish compliance or guarantee an indoor radon result.
The design may also rely on barriers and sealed penetrations. Those measures need continuity where services cross the floor or meet the walls. Coordination is most valuable before the concrete and finishes make the junctions difficult to reach.
Water entry is a separate consideration. Do not treat the cavity as a flood-protection promise or as somewhere that can harmlessly store water. Ground levels, drainage and the actual exposure require their own designed response.
Keep the thermal floor in the comparison
A ventilated void and thermal insulation do different jobs. The project must still define the floor’s insulation and its junctions with walls and supporting elements. CTE DB-HE provides the energy-performance framework for the envelope.
For an international buyer, this distinction matters when reading the word aislado. It may describe separation from the ground in casual conversation without specifying the thermal performance of the completed floor. The product, thickness and junction details should remove that ambiguity.
| Option | What a like-for-like comparison needs |
|---|---|
| Ground-bearing slab | Prepared support, moisture strategy, insulation and finished level. |
| Suspended floor above a void | Designed structure, void conditions, ventilation where specified and access expectations. |
| Permanent formwork cavity system | Base preparation, formers, reinforcement, concrete, connections and the complete floor above. |
There is no universal winner in that table. The right comparison uses the same site information and delivers the same intended rooms, levels and performance.
Translate the proposal into a complete budget
A quotation should identify preparation, structural work, cavity formation, ventilation, moisture measures and insulation. Service routes, external terminations and inspection openings may be separate. The materials and quotation guide helps align supply-only and installed scopes.
Consider a wholly hypothetical 120 m² project. A former-only allowance of €22/m² totals €2,640. A separate installed floor proposal at €110/m² totals €13,200 and includes additional construction work. Those invented figures do not indicate Spanish market prices; they show why the smaller total cannot be interpreted as a saving.
To compare, add the missing tasks to the supply-only offer and identify exclusions from the installed one. Excavation, foundations, insulation, ventilation and taxes should each have a clear place rather than being assumed from the word “complete”.
A slightly higher floor level can also affect entrance details or exterior steps. Keep those consequences in the overall project assessment, even if they fall outside the floor contractor’s package.
Record what becomes inaccessible
Before closing the floor, the project team should record the accepted condition of hidden connections, barriers and service routes. Photographs are more useful when they are labelled against a plan than when they sit in an unstructured phone album.
At handover, locate the ventilation terminals and any designed access points. If you will manage the property from abroad, the maintenance information should make sense to someone who did not watch the work being built.
Describe the ground floor you are planning in Spain, including the municipality, approximate area, ground investigation status and any need for future service access. Mention whether you already have a section drawing or are still comparing options.
Sources and further reading
Sources checked on 19 September 2026. Your property documents and local requirements determine how the guidance applies to your project.
