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Air Quality Mitigation and Ventilation Statement — Grade II Listed Building in Gravesend AQMA
Location: 20 Harmer Street, Gravesend, DA12 2AX, Gravesham Borough Council | Client: Dream Design Develop | Services: Air Quality Mitigation & Ventilation Statement

Protecting Residents from Road Pollution — Without Touching the Listed Frontage

When a planning condition requires mechanical ventilation in a Grade II listed building within an Air Quality Management Area, the engineering has to satisfy two conflicting demands. The ventilation system must deliver clean air to habitable rooms facing a polluted street. And it must do so without altering the external appearance of a protected façade dating from circa 1840.

We were appointed by Dream Design Develop to prepare the Air Quality Mitigation and Ventilation Statement for 20 Harmer Street — a four-storey Victorian townhouse forming part of a uniform stock-brick terrace within the Harmer Street Conservation Area. The statement was required to partially discharge Condition 4(a) of planning permission 20250833, granted on 8 December 2025 for the retention of three one-bedroom flats. A parallel Listed Building Consent (20250842) was issued on the same date.

The Building and Its Context

The property is described in the Historic England listing as having a parapet and dropped eaves cornice between the third and second floors, a stringcourse between the first and second floors, and moulded architraves and cornices above the first-floor sash windows. The upper three floors each contain a self-contained one-bedroom flat: Flat 3 (38.6m² GIA), Flat 4 (38.6m² GIA), and Flat 5 (42.6m² GIA). Each has a combined kitchen/living/dining room at the front facing Harmer Street.

Air Quality Baseline

The site lies within the A226 One-way System Gravesend AQMA, declared for exceedances of the annual mean NO₂ objective of 40 µg/m³. Harmer Street carries significant two-way traffic linking the A226 to Gravesend seafront. The Gravesham Environmental Protection team's standard requirement is that habitable rooms facing the road are fixed shut and served by positively pressurised mechanical ventilation from the uppermost rear point.

The MVHR Solution

A dedicated MVHR unit per flat draws filtered intake air from the rear of the building, passes it through a high-efficiency heat exchanger with acoustic attenuation, and distributes clean tempered air to the front-facing rooms. The system operates in continuous positive-pressure mode at 22 l/s supply and 21 l/s extract in boost mode. Front-facing sash windows are fixed shut with trickle vents sealed or removed. Existing shower-room extract penetrations were re-used as MVHR exhaust terminals where possible, minimising new penetrations through listed fabric.

Heritage-Sensitive Installation

All plant, ducting and acoustic attenuators are located internally. External cowls sit on the rear roof slope, invisible from Harmer Street and powder-coated to match brickwork. Sash windows are retained in their external form with fixing from the inside only. All original architectural features identified in the Conservation Officer's response were retained.

Acoustic targets were ≤30 dB LAeq,T at normal operation and ≤35 dB at boost — below BS 8233:2014 guidelines. Independent commissioning specified per CIBSE Commissioning Code A and BSRIA BG 49/2015.

Outcome

The Statement provided Gravesham Borough Council with the technical detail to partially discharge Condition 4(a), demonstrating that robust air quality mitigation can be delivered within the constraints of a Grade II listed building and a Conservation Area.