Floods and your plot: how to read Q5, Q20 and Q100 before buying
A flood zone doesn’t have to rule out a purchase — but it always means different insurance, a different price and sometimes a building ban. We explain what the Q5–Q100 flows and the active zone mean, and how to check the risk in official maps before you pay a reservation deposit.
A real sample study: parcels in the Droždín cadastral area near Olomouc. Create your own study from an address, a listing or by picking a parcel on the map.
What Q5, Q20 and Q100 mean
Q100 does not mean a flood once every hundred calendar years, but a flow that is reached or exceeded on average once in 100 years. Q5 is a more frequent, smaller flood; Q100 rarer and larger. A plot can lie in Q100 but outside Q20 — the risk is then lower, not zero.
The active zone is a different league
The active zone (aktivní zóna) of a flood area is the part that carries most of the flood flow. Strict building restrictions apply there under the Water Act. A new building in the active zone is usually ruled out; for an existing building expect limits on rebuilding and problems with insurance and financing.
- Does the parcel lie in a Q5, Q20 or Q100 flood zone?
- Does the active zone reach the parcel?
- Is there a building freeze or conditions from the water authority?
- Will a regular insurer cover the property, and on what terms?
- Does the plot price reflect the flood risk?
Where to check the risk
The primary sources are the flood maps and the digital flood plan (VÚV TGM / DIBAVOD data and the POVIS system). Treat third-party maps as a preview only. Always relate the boundary to the specific parcel from the cadastre — a few metres can mean one regime for the house and another for the garden.
Water doesn’t only come from the river
Beyond river floods, check the slope and rain run-off: a plot below a slope can be washed by water from fields, and flash rain floods places far from any stream. Elevation data, terrain slope and neighbours’ experience help. For older buildings, ask about past flooding and water height — and write the answer down.
Real-world example
Why the study separates the parcel from the building
The sample study from Droždín near Olomouc works with exact parcel boundaries from the cadastre and risk layers for each parcel separately. The same principle applies to floods: a flood zone may touch only a corner of the garden while the building stands outside it — or the other way round. That is why Podklad draws risks over the parcel, not over an address point.
Open the Droždín sample studyFAQ
Can I build in a Q100 flood zone?
Outside the active zone it is sometimes possible, but with conditions from the water authority and the building office. In the active zone new construction is usually ruled out.
Why do banks and insurers care about floods?
A flood affects the collateral value and insurability. Insurers may refuse some properties in flood zones or cover them at a much higher premium.
Is the map in the listing enough?
No. Check the parcel in the official flood maps and how the flood boundary relates to the building and the access road.
What if only part of the plot is in Q100?
What matters is where the building and access are or will be. A garden in Q100 with the house outside is a different situation from a house on the edge of the active zone.
Check your own address or parcel
The study preview is free, no sign-in needed. A full study costs 1 credit, no subscription.
Create a free study