Key facts
What the report tells you
What the report tells you — check 22
Is there a radon risk?
It places the building on the CSN radon potential map and checks whether the municipality is in zone I or II of the Building Code. It tells you whether measuring is advisable; it does not measure the radon in your home.
- CSN potential category, from under 100 to over 400 Bq/m³
- Building Code zone I or II, or not listed
- A recommendation to measure when the potential or the zone calls for it
What the municipality's zone means
How to measure radon at home
Order detectors from an accredited laboratory
Find one on the ENAC website; they usually send two.
Place them in the rooms you use
One in the living room and one in a bedroom, or in the basement if you use it.
Leave them for at least three months
Avoiding summer, with the home in normal use.
Send them back and compare with 300 Bq/m³
If you are above it, ventilate more or look into sealing or depressurisation.
What is radon and why is it worth checking?
It is a natural radioactive gas that comes from the uranium in rocks and soil and builds up indoors. It enters from the ground, so it concentrates in basements, ground floors and first floors: according to Spain's Nuclear Safety Council (CSN), above the second floor levels over 300 Bq/m³ are very unlikely.
Radon comes from the decay of uranium found naturally in rock and soil. Outdoors it disperses, but inside a building it can build up, especially where the ground is permeable or rich in radium. The CSN points out that every building has some radon, usually at low levels, and that it is the second cause of lung cancer after smoking.
The floor you live on matters most. Radon comes up from the ground, so it concentrates in basements, ground floors and first floors. According to the CSN, above the second floor it is very unlikely to find levels above 300 Bq/m³.
The CSN radon potential map
The CSN classifies Spain by radon levels and marks the areas where a significant share of homes exceed 300 Bq/m³. Each area's potential is given as the 90th percentile of the readings in its buildings: the value below which the great majority of them fall.
The map identifies areas, not homes. A house in a high-potential area may have little radon and one in a low area quite a lot, depending on its floor, construction and ventilation. You can view it on the CSN website, which also offers the data for download.
What are zones I and II of the Spanish Building Code?
They are the municipalities listed in appendix B of section HS 6 of the Building Code, added by Royal Decree 732/2019, where an unprotected building has a significant chance of exceeding 300 Bq/m³. In zone I, new buildings need a barrier against the ground; in zone II, ventilation or soil depressurisation as well.
Since Royal Decree 732/2019, the Spanish Building Code (Código Técnico de la Edificación) includes section HS 6 on protection against radon. Its appendix B lists the municipalities where, based on CSN measurements, a building constructed without protection is significantly likely to exceed the reference level, and classifies them as zone I or zone II.
In zone I, a new building needs a barrier between the ground and the rooms, or a ventilated air chamber. In zone II, the barrier plus a ventilated containment space or a soil depressurisation system. The rule applies to new buildings, extensions, changes of use and works that affect radon protection; an older building does not have to be adapted unless work is carried out on it.
The reference level: 300 Bq/m³
Article 72 of Royal Decree 1029/2022 sets 300 Bq/m³, as an annual average concentration in the air, as the reference level for homes, buildings open to the public and workplaces. It does not mean a home is uninhabitable; it marks the point at which action is advisable, and only measurements over several months compare with it.
Article 72 of Royal Decree 1029/2022 sets a reference level of 300 Bq/m³, as the annual average concentration in air, for homes, buildings open to the public and workplaces. The Building Code uses the same figure.
It is not a limit above which a home becomes uninhabitable, but the figure that signals when to act. And it is an annual average: a reading taken over a few days cannot be compared with it.
How to measure radon in your home
The CSN recommends long-term detectors, usually nuclear track detectors, left for at least three months and avoiding summer, analysed by a laboratory accredited to ISO 17025; the list of labs can be searched on the website of ENAC, Spain's accreditation body. They usually come in pairs: one in the living room and one in a bedroom, or in the basement if you use it often.
If you are buying, a measurement comes too late to decide but not to negotiate: if the flat is on the ground floor in a high-potential area, you can agree with the seller that it is measured, or who pays for any improvement.
What to do if the level is high
At a few hundred Bq/m³, the CSN says natural ventilation is usually enough; airing the home for about 15 minutes in the morning may do. For higher levels there are building solutions: sealing cracks and service entries in contact with the ground, or installing a depressurisation system under the floor.
After any measure, measure again to confirm it has worked.
What the report does
Check 22 of the report answers “Is there a radon risk?”. It places the building on the CSN radon potential map, gives its category on a colour scale from under 100 to over 400 Bq/m³, and says whether the municipality is in zone I or II of the Building Code. If the potential is medium or high, or the municipality is listed, it suggests measuring.
It does not measure the radon in your home or take your floor into account: only a detector can. The report does not replace an on-site technical inspection of the building. Start with the free report; the Essential report costs €11 and the Complete report €19, with 22 checks on official sources, as a PDF in a few minutes, with a single payment and no account.
Short answers
How do I know if a home in Spain is in a radon area?
Look at the CSN radon potential map and at the Building Code's list of zone I and II municipalities. The HOUSINGFAX report runs both checks from the address and gives you the CSN category and the municipality's zone.
What radon level is acceptable in a home?
Spain's reference level is 300 Bq/m³ as an annual average, set in article 72 of Royal Decree 1029/2022. Above it, it is worth taking steps to bring it down.
How is radon measured at home?
With long-term detectors left for at least three months, avoiding summer, and analysed by an accredited laboratory. Usually two are placed: one in the living room and one in a bedroom.
Does it affect an upper-floor flat as much as a ground-floor one?
No. Radon rises from the ground and concentrates in basements, ground floors and first floors. According to the CSN, above the second floor it is very unlikely to exceed 300 Bq/m³.
What can be done if radon is high?
At moderate levels, daily airing is usually enough. At high levels, seal the entry points from the ground or install a soil depressurisation system. Then measure again.
Official sources you can consult
The links let you review the primary source. Their inclusion does not widen their purpose or turn a piece of context into a diagnosis.
- CSN (Spanish Nuclear Safety Council) — Map of radon potential in Spain (in Spanish)
- CSN — Frequently asked questions about radon in homes (in Spanish)
- BOE (Spanish Official Gazette) — Royal Decree 732/2019, adding section HS 6 on radon protection to the Building Code (in Spanish)
- BOE — Royal Decree 1029/2022, regulation on protection against ionising radiation, article 72 (in Spanish)