Facade retention schemes and their consent trail

Facade retention gets treated as one decision when it is really three consents running on different clocks. The planning permission deals with what the finished building looks like and whether the loss of the structure behind the facade is acceptable. Listed building consent, or the conservation area demolition route, deals with the fabric itself. And the temporary works that hold the wall up while everything behind it disappears carry their own approvals, from a highway licence for the needle beams crossing a footway to the Party Wall awards with the buildings either side.

Get the sequence wrong and the site stops. A contractor who arrives with a design for the propping and no licence for the scaffold on the public footway waits weeks.

When facade retention is the right answer

Facade retention makes sense when the elevation carries the heritage or townscape value and the structure behind does not, and when the wall is in good enough condition to survive being held in the air for a year or more.

It is the wrong answer more often than the industry admits. A rubble-cored wall with lime mortar in poor condition, a facade with large openings and slender piers, or an elevation built as a load-sharing part of the frame behind it, can all cost more to retain than to dismantle and rebuild. Careful dismantling with numbered stones and reconstruction is sometimes the more honest proposal, and it is sometimes the one the conservation officer prefers, though it is rarely the one that reads better in a planning statement.

Test three things before committing:

1. The condition of the fabric, established by opening up rather than by inspection from a cherry picker.
2. The structural role of the wall in the existing building, including whether floors and roof tie into it.
3. Whether the retained elevation can be made to work with the new floor levels, which is where schemes usually fail on interior quality.

Structural design of the temporary support

The support scheme is a piece of permanent-standard engineering with a temporary life. BS 5975 sets out the procedural control expected for temporary works, including the appointment of a temporary works coordinator and design checking proportionate to the risk category, and a facade retention scheme sits at the serious end of that scale.

The engineer has to deal with wind on a wall with nothing behind it, eccentric loads from the wall's own self-weight, the sequencing of demolition behind it, excavation for new foundations close to the props, and the stiffness of the frame relative to the movement the masonry can tolerate. That last point is the one lay readers miss. A frame strong enough not to collapse can still be flexible enough to crack the wall it is protecting, so deflection limits govern the design rather than strength alone.

Two configurations dominate. External propping outside the building line, which is quick to erect and costly in licences and lost footway. And internal propping with needle beams through the openings, which keeps the highway clear and complicates the demolition and the new substructure.

Consents needed for the support itself

The temporary works are development in places where nobody expects it. The recurring items:

• Scaffolding licence under section 169 of the Highways Act 1980 where scaffold occupies the highway, and a hoarding licence under section 172 where the site is enclosed onto the highway.
• Permission from the highway authority for any structure bearing on or over the carriageway, together with any traffic management order needed to suspend parking or close a footway.
• A Party Wall etc. Act 1996 notice and award for work to a party structure, and a notice under section 6 for excavation within the notifiable distances of a neighbouring structure.
• A licence from the adjoining owner for anything crossing their airspace or land, because that is a private right rather than a planning matter. Our note on crane oversailing third-party land explains the same principle for lifting operations.
• Building control approval for the permanent works and, where the temporary works support an occupied structure, engagement with the Building Safety Regulator regime if the building is higher-risk.

The Construction (Design and Management) Regulations 2015 sit over all of it, and the pre-construction information has to describe the retention scheme honestly rather than leaving it as a contractor problem.

Listed and locally listed facades

Where the building is listed, works of demolition, alteration or extension affecting its character as a building of special architectural or historic interest need listed building consent under the Planning (Listed Buildings and Conservation Areas) Act 1990. That catches the retention scheme itself: drilling for needle beams, removing internal structure that the wall relies on, tying the new frame into historic fabric, and making good afterwards.

In a conservation area, demolition of a building or a substantial part of one is relevant demolition requiring planning permission, and carrying it out without permission is a criminal offence. A facade retention scheme normally involves demolishing everything except the facade, so the application has to describe that plainly. Historic England's advice on statements of heritage significance, Advice Note 12, sets out how to establish what matters about the building before proposing to remove most of it.

Locally listed buildings and unlisted buildings that make a positive contribution to a conservation area get weight through the development plan rather than through statute. The practical difference is that the council weighs harm against public benefits without the statutory duties in sections 16, 66 and 72 of the 1990 Act applying in the same way.

One argument to avoid. Claiming that only the facade has value, in a building where the plan form, staircase or roof structure clearly also has value, invites the conservation officer to disagree on the record. Deal with what is being lost, then justify it.

Movement monitoring and trigger levels

Monitoring is how a council satisfies itself that a retained facade will still be there at the end. Expect it to be secured by condition where the building is listed or in a conservation area.

A workable monitoring regime states the instruments, the positions, the baseline, the reading frequency and the trigger levels with the action attached to each. Precise levelling to studs, total station prisms on the facade, tell-tales across existing cracks and inclinometers on the props are the common tools. Trigger levels are usually set in three tiers: a value at which reading frequency increases, a value at which work stops and the temporary works designer reviews, and a value at which the contingency measures are implemented.

Set the values against what the masonry can tolerate, calculated by the engineer, rather than by copying numbers from another job. A lime-mortared brick wall with wide joints accommodates more movement than an ashlar facade with fine joints, and using the same trigger for both is how a monitoring regime becomes theatre.

An illustrative example of how the tiers work. Readings on a four-storey brick facade are taken twice weekly at 12 positions. At 5 millimetres of differential settlement between adjacent positions the frequency moves to daily. At 10 millimetres demolition stops pending review. Those figures are illustrative only, because the real values come from the structural assessment of that wall.

Interfaces between retained and new structure

The junction between old and new is where the cost and the arguments live. Differential movement between a masonry wall on shallow footings and a new frame on piles has to be accommodated, not resisted, and the detail has to allow for the new frame taking load progressively as the props are removed.

Underpinning or a new foundation beneath the retained wall is common, and it interacts with the neighbours' foundations and with the ground investigation. A Geotechnical Assessment that describes the strata beneath the facade, the groundwater regime and the excavation sequence is worth commissioning early, because the propping design depends on it. Where a basement is being formed behind the retained wall, a Basement Impact Assessment will usually be required and, in several London boroughs, is a validation requirement.

Then there are the unglamorous details that decide whether the finished building works: fire compartmentation at the junction, cavity trays and damp proofing behind a solid wall that was never designed to have insulation against it, acoustic separation where the new floor meets the old window head, and thermal bridging at every needle beam pocket.

Evidence for the planning submission

A submission that anticipates the questions gets determined faster. For a facade retention scheme that means, alongside the drawings:

• a heritage statement establishing significance and dealing with the loss of the fabric behind the facade
• a Structural Survey recording the condition of the wall, based on opening up, with photographs
• an outline temporary works strategy showing propping type, extent, and whether it occupies the highway
• a demolition and construction method statement with the sequence set out stage by stage
• a monitoring specification with instrument positions and the principle of the trigger levels
• a construction management plan covering deliveries, footway occupation and hours

Councils commonly condition that demolition may not begin until the temporary works design, the monitoring regime and, in heritage cases, evidence of a contract for the redevelopment have been submitted and approved. Write the application on the assumption those conditions will appear, and the discharge stage becomes a formality rather than a three-month delay.

This is general guidance, not legal advice; check the position with your local planning authority, your building control body and a qualified structural engineer before relying on it. For specialist technical reports, qualified professional input may be required.

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