What the appendix is and where it comes from
The appendix of priority action municipalities is part of the HS 6 section incorporated into the CTE by Royal Decree 732/2019. It gathers the municipal territories where radon potential studies, drawn from the mapping of the Consejo de Seguridad Nuclear (Spain's nuclear safety council) based on geology and available measurements, estimate an appreciable probability of high concentrations in buildings. The list organises municipalities by province and assigns them one of two categories, zone I and zone II, in increasing order of potential.
The resulting geography is that of the substrate: granitic territories and certain formations of the west and northwest of the peninsula dominate, with a notable presence in Galicia, western Castilla y León, Extremadura and the sierras of Madrid, among other areas, alongside scattered municipalities in other provinces. But the operational rule is not to trust geological intuition: the list is nominal, municipality by municipality, and the only valid check is direct consultation of the appendix in its current version.
Zone I and zone II: what changes
The classification is not informative, it is prescriptive: it determines the level of protection required of new buildings and of interventions on existing ones within the section's scope of application. Consulting the appendix belongs at the start of the appointment.
In zone I municipalities, the general requirement is satisfied with a protection barrier between the ground and the habitable rooms, the membrane or equivalent system that limits the entry of the gas, with its continuity and sealing conditions. The barrier's continuity is resolved at the service penetrations.
In zone II municipalities, the barrier alone is no longer enough: the section requires combining it with a second front, either a ventilated containment space interposed between the ground and the rooms, the ventilated crawl space as the typical solution, or a ground depressurisation system under the building, sumps and ducts that capture the gas before it enters and discharge it above the roof. The choice between the two routes depends on how the building meets the ground.
The reference objective in both cases is the same one governing the whole section: keeping the annual average radon concentration in habitable rooms below the reference level of 300 becquerels per cubic metre. The check is made by measurement rather than by calculation.
Practical consequences for the project
| Situation | Design consequence |
|---|---|
| Municipality not on the list | No HS 6 requirement; voluntary measurement possible in bordering areas |
| Municipality in zone I | Protection barrier with its continuity detail and seals |
| Municipality in zone II | Barrier plus ventilated cavity or depressurisation; specific item |
The budget reading is direct. In planned new construction, radon protection is a modest item: the barrier integrates into the ground-contact waterproofing package, and the cavity or depressurisation, where applicable, is resolved with the foundations. The serious cost is that of forgetting: discovering the requirement with the slab already poured turns a minor item into a change order, and in renovation, retrofitted solutions on a finished building multiply the price of what an early detail would have resolved.
Hence the routine this article wants to establish: checking the municipality against the appendix, in the current consolidated version of the document, belongs on the project kick-off checklist, next to the DB-HE climate zone and the DB-HR exterior noise. One minute of consultation, and the requirement sits inside the budget from day one, following the general regulatory traceability principle developed in the article on the cost of CTE compliance.
Two final nuances
First: the list can evolve. Radon potential mapping is refined with new measurements, and CTE revisions can update the appendix; the valid check is always against the consolidated text in force at the project date, not against the archived copy from a previous project.
Second: off the list, the requirement does not apply, but radon exists. In municipalities bordering classified zones, or for sensitive uses with permanent occupation of floors in contact with the ground, prior measurement and the provision of a barrier are quality decisions of marginal cost that some developers adopt voluntarily, and that are at least worth raising with the client.
Note: each municipality's classification and the required solutions are those of the appendix and provisions of the consolidated DB-HS 6 in force, published on the official CTE portal.