What is BOPAS, and does it matter for an extension?

If you read the websites of modular builders for long, you will meet a row of logos: BOPAS, NHBC Accepts, BBA, ISO 9001. BOPAS is the one aimed at mortgage lenders, and the one most often misunderstood.

What BOPAS is for

Banks lend against houses for 25 or 30 years and want to know the building will outlast the loan. With brick and block, they have a century of evidence. With a house made in a factory from timber cassettes or light steel frames, a valuer standing in the hallway has no easy way to judge.

After the problems with some prefabricated housing built in the decades after the war, many lenders became wary of anything “non-standard”. BOPAS was set up to answer that. It was developed by Buildoffsite with the Royal Institution of Chartered Surveyors, Lloyd’s Register and BLP Insurance, in consultation with mortgage lenders. Its central statement is that an accredited system, properly built, will be durable for at least 60 years.

What it checks

The scheme has three parts.

A durability and maintenance assessment. Specialists review the design of the building system: materials, joints, how water is kept out, what will need maintaining and when.

Process accreditation. Auditors look at how the company designs, manufactures and installs. They check that what was approved on paper is what leaves the factory, and that it is assembled correctly on site.

A public database. Accredited systems and the homes built with them are listed, so a valuer or lender can look up a property and see what it is made of.

Accreditation is held by the company for a named building system. It is renewed through repeat audits.

What it does not do

BOPAS is not a warranty. It does not pay out if something fails. That job belongs to a structural warranty or latent defects insurance policy, which is a separate product.

It does not replace building regulations approval either. An accredited system still has to be inspected and signed off, job by job.

And it is not compulsory. Plenty of sound factory-built systems do not hold it, relying on other certification instead.

Does it apply to extensions?

Not directly. BOPAS was written for whole new dwellings, because those are what lenders mortgage. An extension is not sold or mortgaged by itself. It becomes part of an existing house, and a lender valuing that house looks at the property as a whole.

In practice, when you sell or remortgage a home with a factory-built extension, the questions are the ordinary ones: was planning permission needed and obtained, and is there a building regulations completion certificate? A valuer may note the construction of the extension, but a single-storey addition to a conventionally built house rarely troubles a lender.

The picture changes as the factory-built part grows. If you add a full storey, or the extension ends up larger than the original house, a valuer is more likely to ask what it is made of and what evidence of durability exists. Then a maker’s BOPAS accreditation, or an equivalent, becomes useful paper to hold.

So why look for it?

Because of what it says about the company. A maker of modular house extensions that also holds BOPAS accreditation for its housing system has let outside auditors into its design office and its factory, repeatedly. Its drawings are controlled, its materials are traceable, and its installers work to written procedures. The same factory and the same people will build your extension.

Treat it as one good sign among several. It is not a reason to reject a firm that lacks it, particularly a smaller maker that builds only extensions and garden rooms and has never needed to satisfy a mortgage lender.

The other logos, briefly

NHBC Accepts is a review service from the National House Building Council. It confirms that a building system meets NHBC’s technical standards, so homes built with it can have NHBC’s ten-year warranty.

BBA certificates come from the British Board of Agrément. They assess a specific product or system against the building regulations and state how it should be installed. Building control officers know them well.

ISO 9001 is a general quality management standard. It shows the company runs documented processes. It says nothing about buildings in particular.

A structural warranty is insurance. For an extension it is typically offered for ten years and should be insurance-backed, meaning it still pays if the firm has ceased trading.

What to ask a firm instead

For an extension, the documents that protect you are more ordinary than BOPAS.

• Will I receive a building control completion certificate, and who arranges the inspections?
• How will building control see what is inside the walls, given they are closed up in the factory?
• Do you offer a structural warranty? For how long, and which insurer stands behind it?
• Are deposits and stage payments protected if you stop trading before delivery?
• Can I have the structural calculations and the fire and thermal performance data for my file?
• Does your system hold BOPAS, NHBC Accepts or a BBA certificate? May I see it?

A firm that answers the first five well is a safer choice than one that can only point to a logo. Our post on the completion certificate explains why that document comes first.

How long will a factory-built extension last?

Nobody can promise a number for your particular room. What can be said is that the 60-year figure in BOPAS is a minimum design life for assessed systems, the same benchmark lenders apply to new houses generally. A timber or steel frame kept dry will last far longer than that. Durability depends less on where the walls were made than on how well water is kept out at the joints, the roof edge and the junction with the old house. Those are the places to look at when you visit a finished job.

Use the cost calculator for a price range, then get a quote and put these questions to the firm that calls.