Quick answer
A standard composite door threshold sits proud of the floor by around 15-20mm and gives the best weathertightness; a low or level-access threshold reduces that upstand to as little as 15mm or flush, improving accessibility but requiring more careful detailing to keep water out. The right choice depends on who uses the door and how exposed the property is to driving rain.
Overview
Low thresholds are Part M-compliant (15mm max), better for accessibility, slightly weaker on weather sealing. Standard thresholds (50-60mm) seal better but aren't wheelchair-friendly.
Symptoms
- Deciding which to specify.
What actually differs between the two
A standard threshold bar has enough height to house a deep drainage channel and a robust compression seal, creating a physical step that water has to climb before it can reach the inside of the building. A low threshold reduces or removes that step, which is exactly what makes it easier to cross with a wheelchair, pushchair or simply a trip-free step, but it also reduces the physical barrier holding back water.
Manufacturers compensate for the reduced height in low-threshold designs with alternative drainage strategies, tighter-tolerance seals and sometimes canopy or drainage channel additions outside the door, but the underlying physics remains: less height means less margin before water pressure can overcome the seal.
It's also worth factoring in resale and letting considerations: a property marketed as accessible, whether for sale or as a holiday let welcoming guests with mobility needs, can benefit from a genuinely well-executed low threshold as a practical selling point, provided it's been installed to a standard that doesn't introduce a leak risk that undermines the benefit.
Weathertightness performance in practice
BS 6375 sets out how doorsets are classified for weathertightness, including water tightness under simulated wind-driven rain. A well-designed low threshold product can still achieve a good classification under this standard, but the classification is specific to the tested product and its intended installation detail - substituting a cheaper low-threshold system or fitting it without the manufacturer's specified external drainage arrangement risks performance that falls well short of what a standard threshold would achieve in the same spot.
In practical terms, a standard threshold on an exposed elevation is the safer default from a pure weathertightness standpoint, and a low threshold needs more attention to surrounding detail to match it.
Cost isn't the only consideration when weighing the two options - ongoing maintenance differs too. A low threshold's shallower drainage margin means it rewards more frequent cleaning and checking than a standard threshold would need, which is worth factoring into a decision if the household isn't likely to keep on top of that maintenance.
Where Approved Document M comes in
Approved Document M sets accessibility standards for new dwellings and non-domestic buildings in England, including guidance on threshold upstand heights for accessible entrances - broadly favouring thresholds as low and as free of trip hazards as practicable, while still allowing for weather performance. It applies fully to new-build and to specific circumstances such as adaptations for a resident with a disability, and is a common reference point even where it isn't a strict legal requirement for a straightforward like-for-like door replacement.
Some households opt for a standard threshold on the main, weather-facing entrance and a low threshold on a more sheltered side or rear door used for step-free access day to day, which can be a pragmatic compromise where a single door has to serve conflicting priorities.
When a low threshold is the right call
Low thresholds make sense wherever step-free access genuinely matters day to day: a household member using a wheelchair, mobility scooter or walking frame, a home with young children and pushchairs used constantly, or a property being adapted to allow someone to remain living independently for longer. In these situations the accessibility benefit generally outweighs a modest increase in weathertightness risk, provided the installation is done properly with correct external drainage and, ideally, some form of canopy or overhang protection.
It's also worth asking any installer quoting for a low threshold exactly which weathertightness classification the specific product and installation detail has been tested to, rather than accepting a general assurance that 'it's rated for outdoor use' - the difference between a properly tested system and an improvised low-profile conversion can be significant in a genuine storm.
- Wheelchair, mobility scooter or walking frame access
- Frequent pushchair or trolley use through the same door
- Properties being adapted for a resident's changing mobility needs
When a standard threshold is still the better fit
On directly exposed, storm-facing elevations - particularly on Cornwall's north coast and other Atlantic-facing sites - a standard threshold's extra height and deeper drainage channel gives a genuine and measurable margin against wind-driven rain that a low threshold can't fully replicate. Where there's no specific accessibility need, sticking with a standard threshold on the weather side of a property is usually the pragmatic choice, reserving a low threshold for a more sheltered secondary entrance if step-free access is needed elsewhere.
Underfloor heating and floor build-up changes sometimes complicate a low threshold retrofit, since altering floor levels to accommodate a flush threshold can affect how these systems perform, which is another reason a proper survey rather than a like-for-like swap assumption matters here.
Retrofitting a low threshold to an existing doorway
Converting an existing standard-threshold doorway to low-threshold access is possible in most cases but isn't a like-for-like swap - it usually involves adjusting the floor build-up or the external ground level to maintain adequate weathertightness with the reduced upstand, and sometimes altering the door frame itself. It's worth a proper survey before committing, since a badly executed low-threshold conversion can undo any of the accessibility gain by creating an ongoing leak.
For a property that changes hands or changes use over time - for example, a family home that later becomes a holiday let, or a homeowner whose mobility needs change - it's worth thinking about which threshold type gives the most flexibility for future circumstances rather than only the immediate need.
A middle option worth knowing about
Some manufacturers offer a reduced or 'low' threshold rather than a fully flush option, giving a smaller upstand than standard - often somewhere around half the standard height - as a compromise between accessibility and weather performance. This can be a sensible middle ground for a moderately exposed property where full step-free access isn't strictly necessary but a lower trip hazard is desirable.
Building control involvement is occasionally relevant where a threshold change forms part of a larger extension or renovation project, since altered floor levels near an external door can interact with damp-proofing requirements - worth flagging to your installer if the threshold work isn't a standalone replacement.
Ultimately the decision often comes down to a fairly simple conversation about how the door is actually used day to day, weighed against how exposed the property is to the worst of Cornwall's winter weather, rather than a purely technical specification exercise.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Standard threshold, supplied and fitted | From £1,395 | As part of a new composite door |
| Low threshold upgrade on new door | £60-£150 extra | Depending on manufacturer and drainage detail |
| Retrofit conversion to low threshold | £220-£320 | Includes floor/ground level adjustment survey |
| Canopy or overhang addition for protection | £150-£320 | Recommended on exposed low-threshold installs |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- For advice on spec.
Prevention
- Specify correctly first time - retrofit is expensive.
