--- title: "Flood Risk Assessments" description: "Flood risk assessments for planning, from a Stage 1 screening to a Stage 3 hydraulic model, signed by a chartered engineer." updated: "2026-10-12" url: "https://www.petruf.ie/services/civil-engineering/flood-risk-assessments" --- # Flood Risk Assessments > Flood risk assessments for planning, from a Stage 1 screening to a Stage 3 hydraulic model, signed by a chartered engineer. A flood risk assessment shows the planning authority that a development is safe from flooding over its lifetime, climate change included, and that it does not make flooding worse for anyone else. It is asked for where a site sits in or near Flood Zone A or B, where the county development plan's SFRA flags the land, or in a further information request. It goes only as far as the risk does: a screening first, an initial assessment where a risk is found, and a detailed assessment with a hydraulic model where the risk has to be quantified. We carry out all three stages, and the same office designs the drainage and the SuDS the assessment relies on, so the floor levels, the attenuation and the outfall agree with each other when the council reads them. ## What the assessment covers ### River flooding Fluvial flood levels from the OPW's CFRAM and national flood maps, checked against the site's own survey. ### Coastal flooding Tidal levels and the higher sea level allowance, for sites near the coast or a tidal estuary. ### Surface water Rain that ponds or runs across the site before it reaches a drain, which the flood zones do not map. ### Groundwater The GSI groundwater flood maps and the site investigation, where a high water table or a turlough is near. ### Floor levels and freeboard A finished floor level set above the design flood level, with the climate change allowance and freeboard on top. ### Access and escape A safe way in and out during the design flood, which a home needs even where the house itself stays dry. ### Compensatory storage Where building takes floodplain, the volume it displaces given back elsewhere on the site, level for level. ### Hydraulic modelling A Stage 3 model of the watercourse where the risk has to be quantified, including the effect of the development on others. ### Justification Test The development management test for highly vulnerable uses in Zone A or B, answered point by point for the planner. ## Flood Zones A, B and C The 2009 Guidelines for Planning Authorities zone land by how likely it is to flood from rivers and the sea, drawn as if there were no defences and before climate change. The zone and the use together decide what a planning application has to show. ### Flood Zone A: More than 1% a year from rivers, or 0.5% from the sea The highest risk. Homes, schools and other highly vulnerable uses, and less vulnerable uses such as shops and offices, have to pass the Justification Test. ### Flood Zone B: Between 0.1% and 1% a year from rivers, or 0.1% and 0.5% from the sea Moderate risk. Highly vulnerable uses have to pass the Justification Test; less vulnerable uses are appropriate with a flood risk assessment. ### Flood Zone C: Less than 0.1% a year Low risk, and any use is appropriate. A site can still flood from surface water or groundwater, which the zones do not map. ## What you receive Each stage ends in a report the planner can decide on, and a plain answer on whether the next stage is needed. - A Stage 1 screening, with a clear statement of whether more is needed - The design flood level, from the published maps or from a model - A recommended finished floor level, with the climate change allowance and freeboard shown - The route in and out during the design flood - The Justification Test answered, where the use and the zone require it - Mitigation and compensatory storage, designed with the drainage - The report in the form the planning authority expects, and the response if it comes back with further information ## The questions that come first ### Do I need a flood risk assessment for planning? Yes, if the site is in or beside Flood Zone A or B on the OPW maps, if the county development plan's SFRA flags the land, or if the council asks for one in a further information request. Councils can ask for one even where the maps show no risk, since the maps are indicative. ### Do I need one for a house extension? Usually only a short one. The Guidelines treat small extensions as minor development that is unlikely to raise significant flooding issues, so the sequential approach and the Justification Test do not apply, but a flood risk assessment in proportion to the risk should still go with the application. ### Can I build a house in Flood Zone A? Generally not. A dwelling is a highly vulnerable use, so it has to pass the Justification Test: the land must be zoned for it in a development plan adopted under the Guidelines, and the assessment must show the risk is managed on the site and not increased anywhere else. ### Does a 1 in 100 year flood happen once a century? No. It has a 1% chance of happening in any one year, every year. Over 25 years the chance of at least one is about 22%, over 50 years about 39%, and over the 100 years a building might stand, about 63%. ### What are the stages of a flood risk assessment? Stage 1 identifies whether there is a flood risk at all. Stage 2 is an initial assessment that confirms the sources and checks the data. Stage 3 is a detailed, quantitative assessment, usually with a hydraulic model. An assessment stops at the stage that answers the question. ### How do I check if my site is at risk of flooding? Start with floodinfo.ie, the OPW's flood maps and records of past floods, then the GSI groundwater flood maps and the flood maps in your county development plan. They are indicative and none of them maps every source, which is why a site specific assessment is asked for. ### What floor level will the council expect? Above the 1% river flood level, or the 0.5% coastal level, with a climate change allowance added and a freeboard on top for the uncertainty in the figures. The Guidelines set no freeboard figure; 300 mm is common in council flood risk assessments. ### What climate change allowance is used? The OPW's mid range future scenario adds 20% to river flows and 0.5 m to sea level, and its high end scenario 30% and 1.0 m. Which applies depends on the use, and the council's own SFRA usually says. ### How much does a flood risk assessment cost? It depends on the stage. A screening is a short desk study and site visit; a Stage 3 model of a watercourse is a substantial piece of work. Each is quoted before it starts, against the site and what the council has asked for.