Hole Mill's organic swimming pond is the centrepiece of the property — a 3-metre-deep, naturally-filtered, spring-fed body of water with a wooden diving jetty, no chemicals, no chlorine, and an ecosystem of newts, dragonflies, water-lilies and waterfowl that has built up over the years since the pond was first filled.

It was designed and built with help from David Pagan Butler, the Norfolk-based natural swimming pond designer who is, by some distance, the best-known practitioner of low-tech, chemical-free swimming pond design in the UK. This article is partly a guide to what a natural swimming pond actually is, partly the story of how ours was built, and partly a small love letter to the form.

The organic swimming pond at Hole Mill in summer — clear water, the diving jetty, and the marginal regeneration zone planted with water-lilies and reeds
The pond in high summer — the diving jetty extends into 3 metres of spring-fed, naturally-filtered water

What a natural swimming pond actually is

A natural swimming pond — sometimes called an organic swimming pond, a natural swimming pool, or in continental Europe a Schwimmteich — is a swimming-sized body of water that uses plants and biology rather than chemistry to keep the water clear. There is no chlorine, no bromine, no salt-chlorination system, no UV unit, no industrial filter system. The water is kept swimmable by an ecosystem of marginal aquatic plants growing in a dedicated regeneration zone alongside the swimming area.

In rough outline, every natural swimming pond contains two zones:

  • The swimming zone: deeper, clear-bottomed, often around 2-3 metres at the centre. This is the part you actually swim in.
  • The regeneration zone: shallower (typically 30-50 cm), planted densely with marginal aquatics — bulrushes, water mint, marsh marigolds, water lilies, irises, water forget-me-nots and dozens of others. The plants and the gravel substrate around their roots host the bacteria that break down nutrients in the water, keeping it clear.

Water flows passively (or in some designs, gently pumped) from the swimming zone through the regeneration zone and back. The plants pull nutrients out of the water, the bacteria digest organic matter, and over the course of a season an equilibrium establishes itself in which algae has nothing to feed on and the water stays remarkably clear.

The single biggest difference from a chlorinated pool is that a natural swimming pond is alive. There are tadpoles in spring, dragonflies dancing over the water in summer, newts cruising the deep zone year-round, and a thin film of pond skaters on the surface in still weather. You are swimming in a pond, not a sterile box of treated water.

Why we built one

When the four-year restoration of Hole Mill began, one of the more unusual decisions taken early on was to build a swimming pond rather than a conventional swimming pool. The reasoning was straightforward:

  • A chlorinated pool in a Devon valley is an eyesore — a turquoise rectangle in a green landscape, surrounded by safety fencing, with chemicals stored somewhere ugly.
  • A pool needs constant maintenance, chemical balancing, winterising, refilling, and replacement parts. It is closer to a piece of industrial equipment than a feature.
  • A natural swimming pond, by contrast, looks as if it has always been there. It deepens the landscape rather than fighting it.
  • Once established, a properly-built natural pond is dramatically lower-maintenance than a chlorinated pool. There is no weekly chemical regime, no covers to wrestle with, no winter shutdown.

The decision was easy. The harder question was: who do you actually go to for help building one in the UK?

David Pagan Butler

In the small but devoted UK natural-pond-building community, the central figure is David Pagan Butler. He is a Norfolk-based natural swimming pond designer who has been building, designing and writing about natural swimming ponds for over two decades. He runs a popular YouTube channel, has produced an instructional DVD called The Organic Pools DIY Manual, and has written several books on the subject. His approach is distinctive in several ways:

  • He emphasises low-tech, low-energy designs. Many continental Schwimmteich systems use pumps, UV units, and complicated filtration; David's designs typically use minimal pumping and rely heavily on planting and natural water flow.
  • He is a strong advocate of DIY pond building. Most of his work has been about helping ordinary people build their own ponds rather than running a commercial design firm.
  • He has helped build dozens of natural swimming ponds across the UK and Ireland, often visiting sites for a few days, helping shape the design, and then leaving the build itself to the owner and a contractor.

If you are interested in his approach, the most accessible entry points are his YouTube channel and his self-published manual. He demonstrates, with considerable patience, that a natural swimming pond is genuinely buildable on a normal-sized property by anyone with the time and the will.

For the Hole Mill pond, David visited the site, walked the valley with us, advised on the pond's location (using the natural fall of the land to feed water from the springs above), specified the layout of the swimming and regeneration zones, and gave practical guidance on the planting plan.

David Pagan Butler advising on the pond design at Hole Mill
David Pagan Butler on site at Hole Mill, helping shape the pond's design and planting plan

How a natural swimming pond actually works

The science is more interesting than it sounds. The clear water in a natural swimming pond is the result of three overlapping processes:

1. Nutrient stripping by the marginal plants. Algae need nutrients — particularly nitrates and phosphates — to grow. In a chlorinated pool these are kept in check by chemicals; in a natural pond they are kept in check by the marginal plants in the regeneration zone, which absorb nutrients through their roots faster than algae can use them. Starve the algae of nutrients, and the water clears.

2. Bacterial digestion in the substrate. The gravel and substrate around the plant roots host enormous populations of bacteria, particularly nitrifying bacteria that convert ammonia and nitrites into nitrates that the plants then absorb. The biofilm on the substrate is doing most of the actual filtration work.

3. Predation and grazing. A natural pond hosts a food web — daphnia eat algae, fish would eat daphnia (we don't keep fish; many natural swimming pond designers don't), tadpoles graze algae off surfaces, dragonflies eat mosquito larvae. The whole system is dynamic and largely self-regulating.

The result, by midsummer of the second year, is water that is genuinely clear — you can see the bottom 2-3 metres down. It looks and feels like a clean upland lake, not a swimming pool.

The build

The build process for a natural swimming pond, in rough outline, is:

  1. Site selection. Choose somewhere with a level base, ideally with some natural water input — a spring, a stream feed, or even just rainwater catchment. The Hole Mill pond benefits from being fed by the same springs that have been flowing through this valley for centuries.
  1. Excavation. Dig the swimming zone deep (2-3 metres at the centre) and the regeneration zone shallow (30-50 cm). The two zones are separated by a low retaining wall just below the water surface, so water can flow over but plants stay rooted.
  1. Lining. Either with puddled clay (David's preferred low-tech method, used historically across Britain for centuries) or with a heavy-duty EPDM or butyl rubber liner. Puddled clay is more sustainable and ages beautifully but requires the right soil and considerable skill; rubber lining is more reliable for most modern builds.
  1. Substrate and planting. The regeneration zone is filled with washed gravel and planted with a carefully chosen mix of marginal aquatics — typically 15-25 different species. The diversity is the point: different plants thrive at different depths, in different seasons, and contribute different things to the ecosystem.
  1. Filling and stabilising. The pond is filled (slowly, ideally with rainwater or local spring water rather than chlorinated mains water) and then left to stabilise. The first 12-18 months are largely a waiting game while the planting establishes and the bacterial colonies build up.
Constructing the pond — excavation and lining of the swimming and regeneration zones
The build — excavation and lining of the two zones during construction

The first season

The first season of any natural swimming pond is a study in patience. The water turns various colours. Algae appears, persists, sometimes blooms dramatically. New plants take time to establish. Newts, dragonflies and pond skaters appear unbidden, finding the new pond on their own. The bacterial populations slowly build up. By midsummer of the first year the water has typically settled into a clear-but-tinted state. By the second summer it is genuinely clear.

Spring algae — the green tinge that natural ponds typically show in March-May — is completely normal and beneficial. It indicates a healthy, living ecosystem. Most natural swimming pond owners learn to embrace it: swim through it in early spring, break it up, and the system catches up by early summer. The instinct (drawn from a lifetime of chlorinated pools) to "do something" about algae is a hard one to override, but the right answer is almost always to leave it alone.

The pond in spring — early-season algae and plants establishing
Spring at the pond — early algae, water lilies emerging and the marginal planting in full leaf

What it's like to swim in

The honest answer is: completely different from any pool you have swum in. The water is cool (the pond does not warm above about 18-19°C even in high summer; in winter it drops to 4-5°C and is genuinely cold). It is soft — natural water without chlorine has a different feel on the skin, slightly less buoyant, more like swimming in a clean lake. The walls and bottom of the swimming zone are smooth gravel rather than tile. Marginal plants brush past your legs at the edges of the regeneration zone.

The swim itself is properly memorable. Diving from the wooden jetty into 3 metres of clear, cool, slightly-tinted water with the valley sounds around you is a different experience entirely from a length of a chlorinated 25-metre pool. People who swim in our pond for the first time often spend longer in than they intended.

For confident cold-water swimmers, the pond is a year-round resource. Many of our regular guests now combine winter pond swims with the wood-fired hot tub for a hot-cold cycle that is genuinely transformative.

Swimming in the pond — diving jetty and the wider valley
Diving from the wooden jetty into the deep swimming zone at Hole Mill

The wildlife

A properly mature natural swimming pond hosts an extraordinary amount of wildlife. The Hole Mill pond is now home to:

  • Common newts and great crested newts — present year-round, most easily seen in spring during the breeding season.
  • Dragonflies and damselflies — at least eight species recorded, with daily activity from May to September.
  • Frogs and toads — breeding in spring; tadpoles in shoals through April and May.
  • Pond skaters, water boatmen, whirligig beetles — the surface-dwelling and shallow-water invertebrates of any healthy pond.
  • Caddisfly larvae and mayfly nymphs — indicators of clean water.
  • Visiting waterfowl — moorhens, the occasional mallard, herons in early morning hunting for newts.
  • Bats — drinking from the surface at dusk in summer.

This is one of the genuine pleasures of swimming in a natural pond: you are sharing the water with a small community of animals that are all benefiting from it. There is none of the dead, sterile feel of a chlorinated pool.

Pond wildlife — newts, dragonflies and the wider ecosystem
The pond ecosystem — newts, dragonflies, water lilies and the marginal planting in full life

Maintenance through the year

The maintenance regime, once the pond is established, is much lighter than a chlorinated pool's. The annual cycle:

  • Spring: light cleaning of leaf debris from the bottom of the swimming zone. The marginal plants are cut back if they have spread too far.
  • Summer: essentially nothing. Skim leaves off the surface occasionally; check water levels and top up if there is a drought.
  • Autumn: the pond is netted to keep falling leaves out of the water. Leaves rotting in the pond release nutrients that feed algae the following year, so keeping them out is the single most important annual task.
  • Winter: nothing. The pond looks after itself.

There are no chemicals to balance, no covers to wrestle with, no winter closure, no pumps to service (in our case — some natural swimming pond designs include a small pump for water circulation).

A note on cold-water swimming

The Hole Mill pond is open year-round, but swimming is recommended only for confident cold-water swimmers — particularly outside the summer months. The water is cold, deep (3 metres), and there is no lifeguard. Cold-water shock is a real risk for anyone unaccustomed to entering cold water, and we ask all guests to read the cold-water swimming guidance before using the pond.

For the same reasons, we ask that children are always supervised at the pond, and that swimming is never undertaken alone.

Visiting the pond

The pond is available to all guests staying at Hole Mill. There is a small set of pond rules — no dogs in the water (to protect the ecosystem), no suncream or moisturiser before swimming (these contaminate the water), no throwing stones or sticks. The full guidance is on the pond noticeboard at the cottage, and on the pond page on the site.

For anyone interested in building their own natural swimming pond, David Pagan Butler's YouTube channel and DVD are the best place to start. Building a natural swimming pond is genuinely possible for any property with reasonable space and a few months of project time, and it is one of the more rewarding garden interventions you can make.

The Hole Mill organic swimming pond is one of the small, genuinely distinctive features of a stay here — a properly alive body of water in a quiet Devon valley, designed with help from one of the UK's leading natural-pond builders. Check our availability for your dates, or read more about the pond and the wider grounds.