Structural survey for a conversion: scope and limits

A structural survey for a conversion has one job in planning terms: to show that the existing building can become the proposed dwelling without being substantially rebuilt. That is a narrower question than general condition, and the scope of the structural survey for a conversion should be written around it. A report that says the barn is in fair condition and needs some repointing will not answer the prior approval test, and a report that reads as a defect schedule for a buyer will not either.

The test matters because it decides the route. Under Class Q of Part 3 of the General Permitted Development Order 2015, the building operations permitted are limited, and the High Court in Hibbitt confirmed that a structure which is effectively a frame, where conversion means putting up a new building inside or around it, falls outside the class. Most local plan policies on rural building conversion carry a similar requirement in their own words.

What the survey has to establish

Set the brief as a question the engineer answers in a sentence: can this building support the loads of the proposed use, with its existing foundations, walls, frame and roof structure retained, subject to the repairs listed?

Everything else in the report exists to support that sentence. The engineer needs to state, element by element, what is retained, what is repaired in place, and what is replaced. The proportion that falls into the third column is what the planning officer reads first.

Two things the survey should avoid. It should not recommend works that go beyond what the permitted development class allows, because a schedule proposing new foundations and rebuilt walls is evidence against your own application. And it should not hedge every element with a recommendation for further investigation, because an officer reading twelve such recommendations concludes that the building's capability is unknown.

Foundations, walls, roof and floors

Take the elements in the order the loads travel.

Foundations are usually the unknown. Many agricultural and industrial buildings sit on pad bases under a frame, or on a shallow strip under a masonry wall, with no record of either. A trial pit at two or three locations gives the depth, the width, the construction and the founding stratum. Where the proposed use adds floor loading, the engineer needs that information to say anything useful, and a geotechnical assessment may be needed where the ground conditions are doubtful.

Walls come next. For masonry, the questions are the construction (solid, cavity, or single skin), the slenderness ratio against the proposed storey height, the mortar condition, and whether there is lateral restraint at roof level. For a steel or timber frame, the questions are section sizes, connection condition, corrosion or decay at the base, and whether the frame was designed for a sheeted roof rather than a tiled and insulated one.

Roof structure is where agricultural conversions most often fail. A portal frame or a light truss designed for corrugated sheeting carries a fraction of the load of a tiled roof with insulation, a plasterboard ceiling and solar panels. If the roof has to be replaced wholesale with a heavier structure, that is a building operation the class may not permit.

Floors are simpler but not trivial. An unreinforced slab laid for farm vehicles may be adequate in compression and useless as a damp barrier or as a base for underfloor heating. Replacing a slab is normally within the scope of a conversion, so this is rarely the element that decides the application.

Opening up works and when they are needed

A visual inspection cannot tell you what is inside a wall or under a floor. Opening up is the difference between a report that states a capability and one that assumes it.

The usual package is modest:

1. Trial pits to expose foundation depth and width at two or three positions.
2. Removal of small areas of finish to confirm wall construction and the presence of a cavity.
3. Inspection of roof members at bearing points, with resistance drilling or probing where decay is suspected.
4. Opening up at the frame base to check for corrosion below the slab line.
5. Sampling of mortar where repointing or rebuilding is proposed.

Do the opening up before the application rather than after. An officer who reads that the foundations will be investigated at construction stage has no basis for concluding the building is capable of conversion, and a condition cannot cure that because the question goes to whether permitted development applies at all.

On a listed building, opening up works may themselves need listed building consent, so agree the extent with the conservation officer first. Where demolition or asbestos is involved, the demolition and asbestos surveys that support a submission sit alongside the structural work rather than replacing it.

The capable of conversion test

There is no percentage in the legislation. Nobody can tell you that rebuilding 30 per cent of the walls is acceptable and 40 per cent is not, and any report that claims such a threshold is inventing one.

What the decision maker weighs is whether the works are, in substance, a conversion or a rebuild. The factors that come up repeatedly are whether the primary structure is retained and performs structurally in the completed building, whether new structure is being introduced to carry loads the existing structure cannot, and whether the external envelope's position and form are preserved.

So the useful evidence is qualitative and specific. A statement that the existing steel frame carries the new roof without strengthening, that the masonry walls are retained full height with localised rebuilding at three identified positions, and that no new foundations are required, answers the test. A statement that the building is structurally sound does not.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and remember that a Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that works would be lawful.

Loadings for the proposed use

The engineer should state the design loads used, because that is how a reviewer checks the conclusion. Residential floor loading, roof loading including the proposed covering and insulation, snow loading for the site, and wind loading for the exposure category all change when a farm building becomes a house.

Two load changes catch people out. The first is the roof build-up. A warm roof with tiles can be several times the weight of the sheeting it replaces. The second is the introduction of a first floor or a mezzanine, which loads walls that previously carried only the roof and may require lateral restraint that does not exist.

Where a load case cannot be met, say so and quantify the strengthening. Sometimes the answer is that the scheme should be reduced rather than the building strengthened, and it is better to learn that from the engineer than from a refusal.

Limits of a visual inspection

Every structural report for planning should carry a clear limitations section, and it should be read rather than skipped. The honest limits are that concealed elements were not inspected, that no calculations were carried out for elements not opened up, that the ground was assessed at the trial pit positions only, and that the conclusions assume the proposed design remains as drawn.

An illustrative example

A 1960s steel-framed agricultural building with blockwork infill walls is proposed as a single dwelling under Class Q. Trial pits find 900mm deep pad bases under the stanchions in firm clay. The purlins are undersized for a tiled roof, so the design retains the frame and uses a lightweight standing seam covering on new timber rafters spanning between existing purlin positions. The walls are retained with rebuilding at two damaged bays. The report concludes the frame and walls perform structurally in the completed dwelling. This example is illustrative rather than a real case.

Presenting the schedule of works

Present the schedule as a table with one row per element, not as prose. Columns for element, existing construction, condition, proposed action and the reason for it let an officer read the conversion argument in two minutes.

Then write the conclusion in planning language rather than engineering language. The engineer's conclusion is about load paths and capacities. The planning conclusion is that the building is capable of conversion with the works listed, that those works fall within what the class permits, and that no element of the primary structure is being replaced to make the dwelling work. A short Planning Statement is the right place to make that connection, with the structural survey appended as the evidence.

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