Do you need a flood risk assessment for a change of use to an HMO?

Usually, yes. A flood risk assessment is required for a change of use to an HMO wherever the site sits in Flood Zone 2 or 3, and in some Flood Zone 1 cases too. An HMO is still residential sleeping accommodation, so the flood rules that apply to any home apply here. And because more people will live and sleep on the site, councils frequently ask for a drainage strategy as well, even when no extension or extra hard surfacing is proposed. Two questions decide most of it: does the site flood, and where does the extra foul water go.

The change of use does not lower flood risk on paper, but it puts more people on the site, and that is exactly what a planning authority weighs when it asks for a flood risk assessment and a drainage strategy.

The rest of this guide sets out when each is required, why the HMO angle differs from an office or barn conversion, and what a compliant submission has to show.

What counts as a change of use to an HMO: C3, C4 and sui generis

A house in multiple occupation is created when a property is let to people who are not a single household and who share facilities such as a kitchen or bathroom. In planning terms, the use class you are moving to depends on the number of occupants.

• Use Class C4 covers a small HMO shared by between three and six unrelated people.
• Sui generis (large HMO) covers an HMO of seven or more sharers, which sits outside the standard use classes and always needs planning permission.
• The change of use is therefore usually from a single dwellinghouse (Use Class C3) to a C4 small HMO, or from C3 to a sui generis large HMO.

Moving from C3 to C4 is permitted development in much of England, but a lot of councils have removed that right with an Article 4 direction, which forces a full planning application. A move to a large sui generis HMO always needs permission. Either way, once an application is on the table, flood risk and drainage become live considerations. The government's guide to use classes sets out the boundaries in full.

Does a change of use to an HMO increase flood vulnerability?

No, and this is where an HMO differs from most conversions. Under Annex 3 of the National Planning Policy Framework, a single dwellinghouse and an HMO are both classed as "more vulnerable" uses. Converting one to the other does not push the site up a vulnerability band, so it does not trigger the flood tests the way a genuine uplift does.

That is the opposite of an office-to-residential conversion or a barn conversion, where the use moves from "less vulnerable" to "more vulnerable" and flood risk becomes a live issue on that basis alone. With an HMO the driver is not the vulnerability class. It is the intensification of use. More bedrooms means more people to keep safe in a flood, and more wastewater leaving the site.

Our explainer on the flood risk vulnerability classification sets out how NPPF Annex 3 works in detail.

When is a flood risk assessment required for an HMO?

A flood risk assessment is required for a change of use to an HMO in any of these situations:

• In Flood Zone 2 or 3: an FRA is needed for all development, including minor development and changes of use. This is the most common trigger.
• In Flood Zone 1 on a larger or at-risk site: where the site is one hectare or more, or is shown at risk of surface water flooding, sits in a critical drainage area, or is flagged in the council's Strategic Flood Risk Assessment.
• Where another flood source applies: groundwater, sewer or ordinary watercourse risk can bring a Flood Zone 1 site into scope.

These triggers follow the government's flood risk assessment guidance for planning applications, which applies to changes of use just as it does to new build. There is one point worth clearing up, because it trips people up regularly. Changes of use are exempt from the Sequential Test, but that exemption does not extend to the flood risk assessment. A common misreading is to assume that because the Sequential Test is waived, no FRA is needed. It is not. If the site is in Flood Zone 2 or 3, the assessment still stands. Our guides on the flood zones and on when you need a flood risk assessment walk through the triggers in full.

Sleeping accommodation, safe refuge and breach

This is the heart of an HMO flood risk assessment. Because the change of use puts more people asleep on the site, the binding question is rarely whether the building gets wet. It is whether everyone can get out, or stay safe, when it does. In defended tidal locations such as the Humber estuary around Hull and Grimsby, or the tidal Thames, the risk that matters is a breach or overtopping of the defences, and local policy is often explicit that bedrooms cannot sit below the breach flood level.

In practice, that shapes the whole design. A change of use to an HMO beside the Humber has to be modelled against the Environment Agency's breach data for the estuary, with sleeping accommodation kept on the first floor and above, a permanent internal route to those upper floors, and a flood warning and evacuation plan agreed with the council's emergency planners. A Thames-side HMO is assessed the same way against the Thames Barrier breach scenarios, tested against today's conditions and the development's full lifetime with climate change applied. Where floor levels cannot be raised, the case rests on safe refuge, resilient construction and a workable escape plan rather than on keeping the water out. Our guide to residual flood risk explains how breach and overtopping are modelled and mitigated.

Why an HMO change of use also triggers a drainage strategy

This one catches people out. Even where no extension is proposed and the footprint does not change, a change of use to an HMO can still be asked for a drainage strategy, because more occupants means more foul water leaving the site. Some councils spell it out in their validation requirements. The North London boroughs, for instance, follow Strategic Flood Risk Assessment guidance stating that any development which increases the overall impermeable area of a site, or results in an increase in foul water discharge, should be supported by a surface and foul water management strategy.

A drainage strategy can be required on a like-for-like conversion with no building work at all, purely because the extra occupants raise the foul water load on an already stretched sewer network.

We have seen this first hand. A change of use from a single house to a four-person C4 HMO in a London borough was held at validation until a combined foul and surface water drainage strategy was produced. There was no extension and no increase in hard surfacing; the sticking point was purely the rise in foul water discharge. Foul capacity is now one of the earliest constraints a developer meets. The surface water side is handled through the SuDS discharge hierarchy, and the foul side by confirming where the wastewater goes and that the network can take it.

• Surface water: a surface water drainage strategy works down the SuDS hierarchy (reuse, infiltration, watercourse, then sewer as a last resort) and, in London, is summarised on a SuDS proforma.
• Foul water: a foul drainage strategy confirms the discharge point, checks the sewer undertaker has capacity, and evidences that the connection is available and buildable.
• Often one report: government guidance now allows the flood risk assessment and drainage strategy to be combined into a single document, which is usually the neatest route for an HMO.

Plenty of councils ask for the summary on a standardised SuDS proforma at validation, so leaving the drainage work to the last minute can invalidate the application before an officer even reads it.

What your HMO flood risk assessment and drainage strategy must show

A compliant submission for a change of use to an HMO generally works through the following:

1. Identify the flood sources affecting the site: rivers and sea, surface water, groundwater, sewers, and any artificial sources such as reservoirs or canals.
2. Establish the design flood level with the appropriate climate change allowance over the development's lifetime, and set finished floor levels with freeboard where the building allows.
3. Demonstrate safe occupation: safe access and escape, sleeping accommodation above the flood or breach level, and a flood warning and evacuation plan where residual risk remains.
4. Resolve surface water down the SuDS discharge hierarchy, with runoff controlled to an acceptable rate and a maintenance plan.
5. Resolve foul water: confirm the discharge point and that the sewer undertaker has capacity for the extra load the HMO creates.
6. Address residual risk and set out the resistance and resilience measures that make the scheme safe for its lifetime.

Get these right and the flood and drainage elements stop being the thing that holds the application up. Our how-to guide on flood risk assessments sets out the method in full, and the drainage strategies hub covers the surface and foul water side.

If you are planning a change of use to an HMO and want to know whether you need a flood risk assessment, a drainage strategy, or both, Unda can tell you quickly. We prepare flood risk assessments and drainage strategies for HMO conversions across England, deal directly with the Environment Agency and Lead Local Flood Authority on your behalf, and return a fixed-fee quote within 60 minutes on a working day.

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