Introduction
Water is becoming one of the biggest design challenges of our time. Globally, water scarcity is a sustainability issue just as important as energy use, affecting public health, ecosystems, agriculture and food production. 2.1 billion people of the world’s population still lack reliable access to safely managed drinking water.
Although the UK has historically benefited from a relatively secure supply of water, this can no longer be taken for granted. Climate change, population growth, environmental pressures and increasing demand from emerging water intensive industries like data centres and hydrogen production are placing greater strain on water resources. At the same time, heavier rainfall and continued urban development are putting pressure on drainage infrastructure and raising the risk of flooding.
For architects, water design therefore involves much more than providing a supply to a building. It requires consideration of how water is collected, consumed, reused, managed and discharged. Water impacts a whole host including site planning, landscape design, sustainability, flood resilience, Building Regulations, building performance and occupant comfort.
In this guide, we will explore the key principles of water conscious design that every architecture student and designer should understand.
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Designing for Flood Risk
Flooding is an increasingly important consideration for architects and planners. The Environment Agency estimates that around 6.3 million properties in England are in areas at risk of flooding from rivers, the sea or surface water, either individually or in combination.
Flood risk is influenced by several interconnected factors. Climate change is increasing the frequency of intense rainfall and contributing to rising sea levels. At the same time, urban development often replaces vegetation and permeable soil with buildings, roads and other hard surfaces. These impermeable surfaces prevent water from soaking naturally into the ground and exacerbate surface-water flooding.
This is precisely why we need to consider not only whether a building may be affected by flooding but also whether its development could increase flood risk.
Why Flood Risk Is Important
Flooding can occur from several sources:
- River flooding or fluvial flooding occurs when a river exceeds its capacity.
- Coastal flooding may result from high tides, waves, storm surges and rising sea levels.
- Surface-water flooding occurs when rainfall cannot soak into the ground or drain away quickly enough.
- Groundwater flooding occurs when the water table rises and water emerges at the surface or enters below-ground spaces.
- Flooding can also arise from reservoirs and drainage infrastructure.
A site does not need to be located beside a river or the coast to be at risk. Surface-water flooding can affect urban areas when intense rainfall flows rapidly from roofs and pavings into drainage systems. If the capacity of those systems is exceeded, water may collect around buildings, enter properties or flow towards neighbouring land.
Flood risk should be investigated at the earliest stages of a project through site analysis and where required, a Flood Risk Assessment should be conducted. This helps establish the nature of the risk, the suitability of the site and the design measures that may be needed.
Initial site analysis should consider:
- existing and future flood risk
- the topography and natural direction of water flow
- the proportion of permeable and impermeable ground
- existing water sources, drains and sewers
- proposed floor and external ground levels
- safe access and escape routes
- the effect of the development on surrounding properties and land
The Flood Map for Planning is a useful resource for assessing planning risk from rivers and the sea and can indicate whether a Flood Risk Assessment may be needed. It also includes a surface water risk layer.
The Environment Agency’s Check the long-term flood risk service provides broader information about risk from rivers and the sea, surface water and reservoirs.
Understanding Flood Zones
In England, land is divided into Flood Zones 1, 2, 3a and 3b for planning purposes. These zones indicate the probability of flooding from rivers and the sea. They do not provide a complete assessment of other sources like surface water, groundwater or sewers.
They also generally describe flood probability without taking account of the protection offered by existing flood defences.
[Table adapted from The Environmental Design Pocketbook – Sofie Pelsmakers]
Sequential and Exception Tests
Planning policy seeks to direct development towards areas with the lowest probability of flooding. This is known as the Sequential Test. Where suitable lower-risk sites are not reasonably available, certain proposals may also need to satisfy the Exception Test.
The Exception Test broadly considers whether a development offers wider sustainability benefits and whether it can remain safe throughout its lifetime without increasing flood risk elsewhere.
Whether either test is required depends on the flood zone and the vulnerability classification of the proposed use. Flood zones alone do not determine whether a type of development is appropriate.
Flood risk is not the only water constraint on planning permission. In water-stressed areas, some authorities have required water neutrality. This is proof that a scheme will not increase net water abstraction. The best-known case for this is Sussex North (Horsham, Crawley, Chichester), where it stalled thousands of homes from 2021 until the requirement was lifted in late 2025. It is a reminder that water scarcity can block a site even where flood risk is not an issue.
Flood Risk Assessments
A site-specific Flood Risk Assessment may be required for:
- development in Flood Zones 2, 3a or 3b
- sites of one hectare or more in Flood Zone 1
- sites in Flood Zone 1 affected by surface water or other sources of flooding
- sites where future climate change is expected to increase risk
The exact requirements should be confirmed with the relevant local planning authority. Its Strategic Flood Risk Assessment may provide more detailed local information than the national maps.
Further government guidance is available in Flood risk assessments: applying for planning permission.
Flood Resistance and Resilience
The first objective should be to avoid placing vulnerable development in areas of significant flood risk wherever reasonably possible. However, where development in an area of risk is justified, the building and site should be designed to remain safe and to minimise damage.
Flood-risk can influence:
- the position and orientation of the building
- finished floor and external ground levels
- the location of entrances and escape routes
- the use of basements and below-ground accommodation
- structural and material choices
- the location of electrical systems, plant and essential services
- the landscape and drainage strategy
- the movement and temporary storage of water across the site
Flood-resistant design aims to prevent or limit water entering a building. Measures might include flood-resistant doors, sealed service penetrations and appropriately designed walls and floors.
Flood-resilient design assumes that some water may enter and seeks to reduce damage and recovery time. Measures might include water-resistant finishes, raised electrical sockets, accessible construction cavities and layouts that are easier to clean and dry.
The most appropriate approach depends on factors including the expected flood depth, duration, source and speed of flow. Flood-resistance measures are not suitable in every circumstance, particularly where deep water could create damaging pressure differences across the building envelope.
Designing ‘with water’ does not mean accepting avoidable risk. It means understanding where water is likely to come from, where it will flow, where it can be safely stored and how the building can recover if flooding occurs.
Sustainable Drainage Systems (SuDS)
A sustainable urban drainage system (SuDS) allows more water to be absorbed naturally during periods of heavy rainfall and essentially acts as a sponge for surplus water. This reduces the demand on sewers to carry the water away and reduces potential flooding. A SuDS system will also allow the local water table to replenish.
The SuDS method is simple:
- using porous paving
- avoiding impermeable materials
- using soft ground cover
- increasing landscaping and planting
- channeling water to filter naturally into the ground rather than to drains
SuDS are not simply flood-prevention measures. Well-designed systems can also improve water quality, create habitats, support biodiversity, provide attractive landscapes and make public spaces more resilient to a changing climate.
They can be used individually or as part of an integrated site-wide strategy. SuDS are suitable for new developments and may also be introduced during the refurbishment of existing external areas.
There are many different types of SUDS systems:
- green roofs
- rainwater harvesting systems
- permeable paving
- filter drains
- swales
- rain gardens
- soakaways
- detention basins
- retention ponds
- constructed wetlands
- infiltration trenches
The table below outlines how some of them work:
[Table adapted from The Environmental Design Pocketbook – Sofie Pelsmakers]
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Reducing Water Use in Buildings
Approximately 150 litres of water are used per person, per day. Typical water uses are explained in the table below:
[Table adapted from The Environmental Design Pocketbook – Sofie Pelsmakers]
Reducing water demand lowers pressure on public supplies, wastewater infrastructure and the energy required to treat, pump and heat water. It can also reduce running costs and improve the resilience of buildings during periods of drought or restricted supply.
There are many strategies that can be implemented to conserve water, some of which are very simple, others can be costly and more difficult to install. The main principles of water conservation are:
- harvest renewable or local sources
- reduce water usage
- reuse primary supplies where possible
- recycle wastes where suitable
The table gives a useful overview of the different approaches available.
[Table adapted from Rough Guide to Sustainability: A Design Primer]
Water conservation measures should be proportionate to the project. Simple demand reduction measures should be considered first.
More complex systems, including greywater recycling and rainwater harvesting, should be assessed in relation to available space, expected demand, installation cost, operational energy and long-term maintenance.
Let’s explore these further.
Reducing Water Demand
The most effective starting point is to reduce the amount of mains water required by the building.
The key regulatory driver here is Approved Document Part G of the Building Regulations. It sets a maximum of 125 litres per person per day, with a tighter 110-litre optional standard used by many local authorities in water-stressed areas. This is checked via the Water Efficiency Calculator.
Toilet flushing accounts for 25-30% of overall domestic usage. Older toilets can use up to approximately 13 litres per flush, while modern dual-flush toilets commonly use around 4-6 litres for a full flush, with a smaller volume for reduced flushing. The exact saving depends on the toilet being correctly installed and used. Leaking toilets should also be repaired quickly, as they can waste large amounts of water.
A typical bath uses around 80 litres of water. A standard shower may use approximately 10-15 litres per minute, while an efficient shower head may reduce the flow to around 6 litres per minute. A short shower will generally use less water than a bath, but a long shower or high-flow power shower may use as much as, or more than, a bath.
Further reductions can be achieved by specifying low-flow taps, smaller baths and water-efficient washing machines and dishwashers.
In commercial and public buildings, self-closing or sensor-operated taps can help prevent taps being left running, although they need to be correctly set and maintained. Water meters, sub-meters and leak-detection systems can also help identify unusually high consumption or hidden leaks.
Water-saving measures should be considered alongside energy use mainly where hot water is involved, because reducing hot-water demand can also lower carbon emissions.
Key points for reducing water usage:
- Choose water-efficient appliances, sanitary fittings and installations
- Select smaller baths with a lower water capacity
- Avoid high-flow power showers; specify efficient showerheads instead
- Choose appliances with a good current energy rating and low water consumption per cycle
- Consider energy use and carbon emissions alongside potential water savings
- Specify low-flow or aerated taps
- Use self-closing or sensor-operated taps in commercial and public buildings where appropriate
- Install water meters, sub-meters and leak-detection systems
- Repair leaking taps, pipes and toilet cisterns promptly
- Ensure fittings are correctly commissioned and maintained
Wastewater Reuse and Recycling
Wastewater reuse can reduce demand for mains water by treating water for non-potable uses. However, energy use, maintenance, space requirements and whole-life carbon should be considered alongside the water savings.
Greywater recycling
Grey water (which can be described as water from households including shower, bath, bathroom sink, washing machine – not kitchen or toilet waste) can be reused by processing it into usable water for toilet flushing, watering and cleaning purposes.
Grey water must be used immediately after recycling to prevent bacteria build up, or it must be chemically treated. Grey water recycling systems must be considered carefully, as they can create more carbon emissions from pumping and cleaning than using water directly from the mains supply.
Some recycling strategies include:
- Short retention system – a direct collection from the shower or bath and basin, which is fed to cisterns for flushing
- Reedbed – this requires large areas and only suitable for rural areas. Water passes through a UV light prior in order to kill any bacteria. Suffers some water loss through evaporation.
- Biomechanical systems – uses bacteria to break down organic matter in an enclosed system.
They are most suitable in buildings with a steady water demand, like hotels, student accommodation and larger residential developments.
Blackwater and foul water treatment
Foul water is usually flushed away into the sewers for treatment at a sewerage plant. However, it is possible to recycle foul water on site. Foul water treatments must always be designed by a specialist according to specific project requirements.
Some foul water recycling options can be costly and are not always suitable for the site, however, some of the options are listed below.
- Compost toilet – a waterless toilet that allows the natural process of waste into compost (not suitable for urban or public buildings)
- Holding tanks – a septic tank that gradually separates the solids from the wastewater so it can be treated.
- Package treatment plants – compact mechanical and biological treatment systems.
- Living machine – this is a natural process where vegetation is placed on hydroponic tanks that treat and recycle the wastewater. It is quite costly and not widely used at the moment.
- Reedbeds – these systems treat and recycle wastewater through natural processes using a biological ground system. Not suitable for urban areas.
Often the above options are selected due to remote locations where connection to a mains sewerage system is difficult.
Rainwater Harvesting
Rainwater harvesting collects runoff, usually from roofs and stores it in a tank located in the ground or basement for later non-potable use. A simple filter and pump can allow the water to be used for flushing toilets, washing and cleaning and for watering gardens. Harvesting rainwater can also reduce the load on the wastewater systems.
Detailed design calculations will need to be carried out to predict rainfall, runoff coefficient and tank storage size along with pump requirements. Some problems do exist with the rainwater harvesting system. These include:
- Storage space for water tanks can be limited. New build construction can design this into the scheme but in existing buildings the weight of stored water can make the option prohibitive.
- Quality of rainwater can vary according to area and treatment and generally will not be suitable for drinking
- Initial construction costs to implement a rainwater harvesting strategy can be costly and capital may not be recovered quickly.
- The building will need to be designed to maximise water catchment, which could have an aesthetic consequence.
It is suitable for buildings with large roof areas and consistent non-potable demand. This includes schools, offices, industrial buildings, apartment developments and buildings with landscaped grounds.
Embodied/virtual water in materials
It is also worth noting embodied/virtual water which is consumed in producing materials like concrete, brick and steel is distinct from the operational water covered above. It is a less established metric than embodied carbon but material choice and reuse of existing structures both carry a water dimension worth flagging.
Water Design Checklist for Architects
You can use this checklist to address key water considerations at each project stage.
Concept Stage
- Review flood maps and identify the site’s flood risk
- Identify all potential sources of flooding, including surface water
- Understand existing site topography and discharge routes
- Locate existing water sources, drains and sewers
- Identify the proportion of permeable and impermeable ground
- Review relevant planning and environmental constraints
- Confirm if specialist advice is needed
- Identify early opportunities for sustainable drainage systems (SuDS)
- Consider how future climate change may affect flood risk, rainfall and water demand
- Consider the potential impact of the development on neighbouring land
Design Development
- Develop an integrated drainage and landscape strategy
- Follow the drainage hierarchy: reuse, infiltration, controlled discharge
- Coordinate drainage proposals with civil engineers and landscape architects
- Avoid locating vulnerable uses, plant and electrical services in high-risk locations
- Incorporate proposed Finished Floor Levels (FFL) and external ground levels into the site layout
- Incorporate appropriate SuDS measures
- Reduce potable water demand through efficient design
- Specify water-efficient fixtures and fittings
- Assess the feasibility of greywater recycling
- Assess the feasibility of rainwater harvesting
- Provide safe access for inspection, maintenance and replacement
- Design systems with sufficient capacity and resilience for future climate conditions
Before Planning Submission
- Confirm whether a Flood Risk Assessment is required
- Confirm whether the Sequential or Exception Test applies
- Prepare or coordinate the drainage strategy
- Demonstrate compliance with water-efficiency requirements
- Confirm the proposed SuDS approach and discharge hierarchy
- Coordinate the landscape and drainage strategies
- Confirm maintenance responsibilities for drainage and water systems
- Ensure proposals are consistent across drawings, reports and consultant information
Helpful Links
Approved Documents – Approved Documents – Planning Portal
NHBC Foundation – NHBC
The Water Efficiency Calculator for New Dwellings
Flooding – GOV.UK
Check the long term flood risk for an area in England – GOV.UK
Get flood risk information for planning in England – Flood map for planning – GOV.UK
Risk of flooding from surface water – understanding and using the map – GOV.UK
Flood risk assessments: applying for planning permission – GOV.UK
Check if the sequential and exception tests apply – Understanding and assessing flood risk – Planning Portal
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Conclusion
To summarise, water should be considered as an important part of architectural design from the very beginning of a project. How we manage flood risk, drainage, water use and reuse can directly affect the resilience of our buildings.
By using strategies like sustainable drainage systems (SuDS), water-efficient fittings, rainwater harvesting and flood-resilient design, we can reduce pressure on water supplies and drainage infrastructure while creating places that are better prepared for climate change.
We hope this post helps you consider water more carefully in your next project.
Don’t forget, you can read all the posts in our Architecture and Environment Series here:
Architecture and the Environment
Thank you for reading!
Your Comments
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Authors
Emma Walshaw, Architectural Technologist and founder of First In Architecture and Detail Library. Emma has written a number of books about construction and architectural detailing.
Valanne Fernandes, a Part 2 Architecture graduate. Valanne is a content creator with First In Architecture, spending her time researching, writing and designing inspiring new content for the website.
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