Modern open-plan living brings fantastic light and space, but it often creates serious indoor air quality and moisture control challenges. When fresh air supply and extraction are overlooked, the side effects extend far beyond damp walls and musty odours. Occupants frequently experience tangible health and comfort impacts, including headaches, poor sleep quality, daytime fatigue, and reduced concentration. Designing proper ventilation from the outset is vital, especially when extensions connect fluidly to the rest of the property.

A recent case study in Sevenoaks highlights how overlooking these ventilation fundamentals at the design stage can compromise a brand-new living space.

The Sevenoaks Case Study: A Decentralised MVHR Disconnect

In this project, the client faced persistent issues with their newly built extension. They had installed a decentralised mechanical heat recovery unit, but several critical design and installation errors rendered it ineffective:

• Poor Unit Location: The unit was placed right next to a window featuring a trickle vent and inside a restricted side passageway. This external positioning caused dead air and turbulence issues.
• Inadequate Specification: The unit capacity and specification simply could not handle the volume of the new extension, particularly because the space was completely open to the rest of the house after the dividing doors shown on the original architectural plans were omitted.
• Authorised Builder Deviations: The builders altered the kitchen layout and relocated both the kitchen and the extractor to different positions than those specified on the architect’s plans.

This scenario is remarkably common. Builders frequently vary work from architectural drawings on site without ensuring the client consults the original designer. That single omission can ruin the usability and lived experience of the new area, turning a dream open-plan kitchen into a stagnant moisture trap.

The Homeowner Role: Protecting the Design Intent

As a layperson navigating a build, it is vital to ensure you make full use of your designer throughout the construction process and verify that any deviations made by the builder are formally approved by them first. You should also ask your designer to clarify whether additional building regulation compliance advice or specialised specification is required in respect of building services before work proceeds on site.

Key Ventilation Design Principles for Modern Extensions

To avoid these pitfalls and protect indoor air quality, several critical technical factors must be addressed early in the design process:

• Establish Moisture Sources Early: Identifying the precise location of moisture generation sources such as cooking zones and utility areas early on ensures that extractors capture pollutants at the source before they migrate across open-plan spaces.
• Compartmentalisation: Depending on the scale and layout of the extension, compartmentalising the space may be necessary to control airflow paths and prevent moisture-laden air from damaging adjacent historic or modern structures.
• Architectural and Structural Obstructions: Features like downstand beams and large roof lantern lights disrupt normal airflow patterns, creating stagnant pockets where condensation and mould can readily form.
• Ducting and Placement Realities: Extractor performance is frequently much lower than manufacturer-stated laboratory figures. The choice of rigid versus flexible ducting, combined with restrictive locations like external passageways where dead air and turbulence occur, drastically reduces real-world extraction rates.
• Choosing Systems Early: Mechanical extraction or mechanical heat recovery must be designed into the fabric of the building at the initial design stage, rather than retrofitted as an afterthought.
• Location and Exposure: Consideration must be given to the geographic location, local exposure ratings, and the building’s natural ventilation tendencies to balance airtightness with adequate fresh air supply.
• Realistic Usage Patterns: Ventilation design must account for how the extension will realistically be used by the household day-to-day, rather than relying on theoretical occupancy numbers.

Regulatory Compliance and Commissioning

Residential ventilation in the UK is governed strictly by Approved Document F: Ventilation, of the Building Regulations, alongside Approved Document C: Site preparation and resistance to contaminants and moisture. Building regulations set mandatory minimum extraction rates for intermittent and continuous extractors, as well as whole house ventilation standards. Ensuring compliance requires calculating the volume of the space and accounting for pressure drops across duct runs, which safeguards both the structural fabric of the building and the health of its occupants.

To guarantee that a system performs as intended in practice, it is best to ensure actual extraction performance is tested on site and that formal commissioning is undertaken and certificated, which is often a strict requirement of Building Control sign-off.

How We Can Help With Damp, Mould, and Air Quality Issues

If you are suffering from damp, mould, or air quality issues in your property or new extension, we can help. Through our targeted defect investigation and analysis services, we provide a specific defect report that clarifies why these issues are occurring and outlines practical options to resolve them.

About the Author

This technical insight was prepared by Timothy Hicks MCIOB AssocRICS, an experienced building surveyor specialising in defect diagnostics and structural investigations.