Do you need a flood risk assessment for an office-to-residential conversion (Class MA)?

Often, yes: a flood risk assessment for an office-to-residential conversion is required whenever the building sits in Flood Zone 2 or 3, because Class MA permitted development still has to clear prior approval, and flooding is one of the matters the council must weigh. The change from commercial to residential use raises the building's flood vulnerability, so the local planning authority needs a site-specific assessment before it can agree the change. This guide sets out when an FRA is needed, what it has to show, and how the prior-approval process treats flood risk. Where a building touches a mapped flood zone, a flood risk assessment for planning is the document that clears the flooding hurdle.

Turning offices into homes under Class MA does not sidestep flood risk. If the building is in Flood Zone 2 or 3, expect to submit a site-specific flood risk assessment with the prior-approval application.

Do you need a flood risk assessment for a Class MA conversion?

If the building is in Flood Zone 2 or Flood Zone 3, you will almost always need a site-specific flood risk assessment to secure Class MA prior approval. Planning Practice Guidance requires prior-approval applications in areas at risk of flooding to be accompanied by an FRA, and it requires the council to consult the Environment Agency where the site falls in Flood Zone 2 or 3. In Flood Zone 1 an FRA is not usually needed, unless other sources of flooding are in play. The August 2026 National Planning Policy Framework, in force since 17 August 2026, carries this through: its Policy F4 makes a site-specific flood risk assessment a national information requirement for development in Flood Zones 2, 3a and 3b.

The flood zone is the first thing to check. The Environment Agency's Flood Map for Planning shows which zone a building sits in, and it is the same map the council will use. If any part of the site is in Zone 2 or 3, treat an FRA as expected rather than optional, and it is worth getting a flood risk assessment for planning scoped before you apply so the flooding matter is settled at the first attempt.

What Class MA actually allows (and what changed in 2024)

Class MA is the permitted development right that lets a building change use from Class E (commercial, business and service) to Class C3 (dwellinghouses). It came into force on 1 August 2021, and it is not a free pass: the change is permitted development, but only once the council has granted prior approval. In practice that makes it a lighter-touch route than a full planning application, not an exemption from scrutiny.

The rules changed materially on 5 March 2024. The amending order removed both the 1,500 square metre floorspace limit and the requirement for the building to have been vacant for three months, so larger office blocks and occupied buildings can now come forward. The one qualifying condition that remains is that the building must have been in Class E use for at least two years before the application.

• Class E to C3. The right converts commercial, business and service premises, including offices, shops and cafés, into homes.
• Prior approval is mandatory. The council assesses a defined list of matters before the change can proceed.
• No size cap, no vacancy rule. The 1,500 square metre limit and the three-month vacancy requirement were both removed in March 2024.
• Two years of Class E use. The building must have been in a qualifying commercial use for at least two years before the application.

Class MA is also blocked on certain protected land: sites of special scientific interest, listed buildings, scheduled monuments, safety hazard and military explosives areas, National Parks, the Broads, National Landscapes (AONBs) and World Heritage Sites. Conservation areas are different. Unlike Class Q, which is barred in conservation areas, Class MA is permitted there, subject to an extra prior-approval consideration about the loss of ground-floor commercial use. Our guide to the flood risk assessment for a barn conversion (Class Q) covers the parallel route for agricultural buildings.

Flooding as a prior-approval matter

Flooding is written into Class MA as one of the matters the council must consider. The legislation lists "flooding risks in relation to the building" alongside transport, contamination, noise, natural light, fire safety and the other prior-approval considerations. The statutory wording is not limited to any particular flood zone; the flood-zone trigger comes from national guidance, and that guidance is explicit.

So the flood risk assessment is the document that discharges the flooding matter. Without it, the council has nothing on which to agree the change, and an application in a mapped flood zone is likely to stall or be refused. The prior-approval matters the council weighs are these.

• Transport and highways. The impact of the development, particularly safe access to the site.
• Contamination. The risk of land contamination affecting future residents.
• Flooding. Flooding risks in relation to the building. This is the matter a site-specific FRA addresses.
• Noise. The impact of noise from nearby commercial premises on the new homes.
• Natural light. Whether all habitable rooms receive adequate natural light.
• Other matters. Fire safety, the effect of losing a nursery or health centre, industrial-area impacts, and, in a conservation area, the loss of ground-floor use.

When exactly does a Class MA conversion need an FRA?

The short answer is that flood zone decides it. In Flood Zone 2 or 3 a site-specific flood risk assessment is expected and the Environment Agency will be consulted. In Flood Zone 1 an FRA is not normally required for the change of use on its own, but Policy F4 now catches more Zone 1 sites than the old rules did: an assessment is needed where the site is one hectare or more, where the Environment Agency has flagged critical drainage problems, or where the Flood Map for Planning or the strategic flood risk assessment shows the land at risk from any source. Since the May 2026 map update added surface water climate change layers, that now catches town-centre buildings the earlier maps left clear.

The decision follows the flood zone the building sits in, so it is worth confirming that first on the Flood Map for Planning. The table below sets out the position by zone.

Even where an FRA is not strictly triggered, a short screening note explaining why can save time, because it lets the council close out the flooding matter without a request for more information. Our explainer on when you need a flood risk assessment works through the triggers in more detail.

Why office-to-residential raises the flood stakes

The reason flooding matters so much for these conversions is the change in vulnerability. Under the National Planning Policy Framework, an office is a "less vulnerable" use and a dwelling is a "more vulnerable" one. Converting from one to the other moves the building up the flood vulnerability scale, and that is what makes flood risk a material consideration even where the building itself is not changing. The August 2026 NPPF brought the vulnerability classification and the flood-zone compatibility table into the Framework itself, in Annex F, but it left the office-to-dwelling shift exactly where it was.

A conversion that looks modest on paper, from a dry office to a set of flats, can move a building from "less vulnerable" to "more vulnerable" in flood-risk terms, which is exactly why the assessment is needed.

The concern is sharpest where people would sleep at ground or basement level. Think of a town-centre office in a river or tidal flood zone. As a workplace that empties at night it may be perfectly acceptable, yet the same building can pose a real risk once it houses ground-floor or basement flats, where occupants could be trapped or cut off during a flood. This matters most for surface water: recent claims analysis found that seven in ten UK home insurance flood claims relate to surface water from sudden, intense rainfall rather than rivers bursting their banks, a source that concentrates on exactly these urban infill sites. Our explainer on the flood risk vulnerability classification sets out how those categories work, now consolidated into Annex F, Table 2, and why the shift from commercial to residential matters.

Does the sequential test apply to a change of use?

Not usually. The August 2026 NPPF now writes this exemption into national policy: Policy F5 takes most changes of use out of the sequential test, and a Class MA conversion to a dwellinghouse is one of them. (The exceptions are changes of use to a caravan, camping or chalet site, or a mobile home or park home site, which Class MA is not.) The exception test, now Policy F6, is confined to river and sea flooding. So a Class MA prior approval sits outside both tests, which is often misread as meaning flood risk can be set aside, the same trap that catches barn conversions under Class Q.

The sequential test and the flood risk assessment are two different things. Skipping one does not skip the other.

That misreading does not hold. Even when the sequential test does not apply, the requirement for a site-specific FRA at prior approval stands, because it flows from the prior-approval process and from national guidance, not from the test. The conversion skips the exercise of steering development to lower-risk land, but it still has to prove the homes themselves can be made safe. Where a test genuinely does apply to your wider scheme, Unda also prepares sequential and exception test reports, and our note on what the Yatton appeal means for the sequential test unpacks how it is applied in practice.

What your flood risk assessment must show

A site-specific FRA for a Class MA conversion does the same core job as one for a full application: it shows the council and the Environment Agency that the new homes can be made safe for their lifetime, the test Policy F7 sets for any development in a flood risk area. In practice it needs to cover the following.

• Flood sources and probability. All sources affecting the site (river, tidal, surface water, groundwater and sewer) and the flood zone the building sits in.
• Design flood level with climate change. The modelled flood level for the appropriate return period, uplifted for the current climate change allowances over the development's lifetime.
• Finished floor levels. Habitable floor levels set safely above the design flood level, typically with 300mm of freeboard where the layout allows it.
• Safe access and escape. A dry or low-hazard route in and out, and an assessment of whether occupants can leave or shelter safely during a flood.
• Surface water and drainage. How surface water will be managed so the conversion does not increase runoff, with any sustainable drainage designed to the 2025 National Standards under Policy F8, addressed in line with surface water flooding and planning.
• Residual risk and resilience. Flood-resistant and resilient construction, a flood warning and evacuation plan, and a clear statement of the risk that remains after mitigation.

For a fuller walk-through of the method, see our guide on how to do a flood risk assessment. Raising floor levels is not always possible in a tight town-centre building. Where it is not, the case rests more heavily on resilient construction, safe refuge and a workable evacuation plan, and any unfamiliar term, from freeboard to functional floodplain, is defined in our flood risk and drainage glossary.

How the council and Environment Agency decide

Once the application is in, the council has 56 days to determine the prior approval. Where the site is in Flood Zone 2 or 3 it must consult the Environment Agency, which will comment on the flood risk assessment, often drawing on its published standing advice. A clear, well-evidenced FRA keeps that consultation short and the decision on track. A missing or weak one is the usual reason a flooding matter drags out or is refused, and the culprit is often something small: a climate change allowance left out, or floor levels that do not tally with the site survey.

If the 56 days pass without a decision, prior approval can be deemed to be granted, but only where the application was valid and complete. That is another reason to get the flood risk assessment right first time rather than treat it as something to add later. Our overview of planning with the Environment Agency's flood risk standing advice explains how the consultation works.

It is not worth cutting corners and converting first. A building brought into residential use without addressing flood risk is exposed to enforcement and to the difficulty of regularising it later, as we set out in our note on retrospective planning and flood risk. If you are weighing up a Class MA conversion and the building touches Flood Zone 2 or 3, it is worth getting the flood risk assessment for planning scoped early, and we can tell you quickly whether your site needs one at all.

Frequently asked questions