Walkthrough: a car park converted to housing

A surface car park developed for housing is one of the better brownfield propositions a small developer will see, and the application turns on two things: whether the loss of the parking can be justified with survey evidence, and whether a wholly impermeable site can be drained to the standard a major application now requires. The contamination position is usually straightforward and the design argument is usually about boundaries.

What follows is an illustrative walkthrough of a scheme of nine flats on a privately owned car park behind a high street. The site, the figures and the officer responses are composed to show how the application runs.

Establishing the lawful use and the ownership

The first week went on title and planning history rather than design. Three questions had to be answered before anything was drawn.

What is the lawful use of the land. A car park may be sui generis as a public car park, or ancillary to a shop or office, or it may have no planning history at all because it predates the requirement. The planning history search found a 1974 permission for a car park serving a parade of shops, which made it ancillary and raised the question of whether removing it prejudiced the shops.

Who owns it and who has rights over it. The title showed rights of way in favour of three of the shop units and a right to park for one of them. Those are private rights, and planning permission does not override them, which meant the layout had to accommodate the parking right or the right had to be released by agreement. That took longer than the application.

What is the ownership certificate position. With third party rights and an access strip in different ownership, a certificate B with notice served on the other owners was needed rather than certificate A.

Loss of parking policy and the survey evidence

The adopted local plan protected parking serving a defined local centre unless it could be shown to be surplus. Eleven objections arrived from traders.

The evidence that answered it was a Parking Survey carried out to the accepted methodology: occupancy counts of the car park itself and of the surrounding on-street parking, taken on multiple weekdays and a Saturday, at hourly intervals from 08:00 to 20:00, plus overnight counts for the residential streets. The survey covered two separate weeks in a neutral month, avoiding school holidays.

The results did the work. The car park ran at under half occupancy at its busiest hour, the on-street network had spare capacity within 200 m at all surveyed times, and a public car park 150 m away had substantial spare capacity throughout. The scheme then retained four spaces at the rear for the shop rights and provided no resident parking at all, with the application supported by a car free justification: a town centre location, a railway station within 600 m, and secure cycle parking at a ratio above the authority's standard.

The highway authority asked for a controlled parking zone contribution to manage overspill, secured by obligation. Accepting it was quicker than arguing about it. Transport evidence at a larger scale is covered in a school expansion and the transport evidence.

Contamination from the former surface use

A car park is a low risk site chemically, and the risk that matters is localised rather than widespread: hydrocarbon and oil staining, any former fuel storage, and made ground of unknown origin beneath the tarmac.

The desk study identified the site as part of a larger yard used for vehicle repairs until the 1960s, which changed the picture. A Land Contamination Phase 1 desk study and site walkover produced a preliminary conceptual model with a credible source, a pathway through made ground and a receptor in future residents and garden soils. That recommended intrusive investigation.

Phase 2 followed by condition rather than before determination: window samples on a grid with soil chemistry, ground gas monitoring over the required number of visits, and a remediation strategy. The investigation found a localised hydrocarbon hotspot where the repair pit had been, dealt with by excavation and disposal, and a clean cover system to the garden areas.

Both phases require preparation by suitably qualified and experienced persons in accordance with the relevant British Standard, and the local planning authority expects that. An AI-assisted draft can organise the desk study and the conceptual model; the investigation, interpretation and remediation strategy are for a qualified professional.

Drainage on a wholly impermeable site

The counter-intuitive part of the scheme, and the one the engineer enjoyed. A fully tarmacked car park has an existing run-off rate close to 100 per cent of rainfall, so almost any residential redevelopment with gardens and planting reduces run-off.

That gave the drainage case a strong starting point, and the lead local flood authority still wanted the full package. A Drainage Strategy set out the existing impermeable area and its calculated run-off, the proposed permeable and impermeable areas, infiltration testing results showing the underlying clay was unsuitable for soakaways, and attenuation sized for the design events including the climate change allowance.

The solution was a cellular attenuation tank beneath the courtyard with a flow control discharging to the public surface water sewer at a rate well below the existing, permeable paving to the parking and access, a rain garden to the frontage, and water butts. A betterment of more than half the existing rate was achieved, which is what secured the consultee's support.

Exceedance routing mattered because the site fell towards the rear boundary and the neighbouring gardens. The drawings showed overland flow held within the courtyard by a raised threshold detail and finished floor levels set 150 mm above external ground.

Design, overlooking and the boundary relationships

A car park behind a high street is enclosed by other people's rear boundaries, so the design constraint is overlooking in several directions at once.

The scheme ran as a three storey L shaped block along two boundaries with a courtyard, which put habitable rooms at the back of the site facing the rear gardens of houses on the next street. The original layout had second floor windows 14 m from those boundaries.

Changes that made it acceptable: habitable rooms rearranged so bedrooms rather than living rooms faced the closest boundary, second floor windows to that elevation reduced to high level obscure glazed rooflights, the top floor set back by 1.8 m to reduce the apparent height from the gardens, and a 45 degree assessment from the nearest neighbouring window submitted with the plans. An Overshadowing Study tested the effect on the two most affected gardens and showed the retained sunlight met the recommended levels.

Boundary treatment carried more weight than expected. A 2 m close boarded fence on a brick plinth with planting on both sides answered more objections than the window changes did.

The documents submitted

The pack for a nine unit scheme on a brownfield town centre site:

• Forms, certificate B with notices served, and the fee.
• Location plan, existing site plan showing the car park layout, proposed site plan, floor plans, elevations, sections and a street elevation.
• Planning statement covering the brownfield policy position, the loss of parking, and housing need.
• Design and access statement with the 45 degree assessments and the boundary sections.
• Parking survey and a transport statement with the car free justification.
• Land contamination Phase 1 desk study.
• Drainage strategy with infiltration results and attenuation calculations.
• Biodiversity net gain assessment, with the baseline measured on a site that was almost entirely hardstanding.
• Overshadowing study and a daylight note for the proposed units.
• Arboricultural information for two mature trees on the boundary in neighbouring ownership.

Biodiversity net gain was the easy win. A site of tarmac has a very low baseline, so modest planting, a green roof on the bin store and a courtyard of shrubs and small trees comfortably exceeded the required gain on site. The statutory metric still has to be completed by a competent person under the Environment Act 2021.

Conditions and the levy position

Permission came with conditions requiring Phase 2 investigation and remediation before works start, a verification report before occupation, the drainage scheme and its maintenance plan, obscure glazing retained in perpetuity, the top floor setback as approved, boundary treatment before occupation, cycle parking, and the four retained spaces kept available for the shop units.

The community infrastructure levy position needed care. The scheme was liable on its net additional floorspace, and there was no existing building to deduct because a car park has no floorspace, so the full new floorspace was chargeable. Check the charging schedule and the relief position before the appraisal is finalised, because the rates are set by each authority and indexed annually.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. Enter your postcode on PlanScope to see which reports your site is likely to need.