Questions to ask about an extension or loft conversion

Why altered properties need more scrutiny

When you buy a property that was built by a developer, it was subject to planning permission, building control inspections throughout construction, and (for newer properties) a structural warranty. When you buy a property that has been altered by a homeowner, those safeguards may or may not have been followed.

The most common issues buyers encounter with altered properties are:

• No planning permission - the work was done without the required consent, or under permitted development rules that didn't actually apply
• No building control sign-off - the work was never inspected for structural adequacy, fire safety, or compliance with Building Regulations
• Poor workmanship - the physical quality of the build is substandard, creating ongoing maintenance liabilities or structural risk
• Design problems - the extension or conversion doesn't integrate well with the original house, creating awkward layouts, poor natural light, or thermal bridging

None of these will necessarily be visible in estate agent photos or apparent from a quick walk-through. You need to ask the right questions and inspect carefully. For a broader viewing framework, see our house viewing checklist.

Planning permission and permitted development

The first question for any extension or conversion is: was planning permission obtained?

Some extensions and alterations can be carried out under permitted development rights (PD) - a set of automatic planning permissions that allow certain works without a formal application. Common PD works include single-storey rear extensions (up to 3m for attached houses, 4m for detached), loft conversions adding up to 40m³ of space (50m³ for detached houses), and some outbuildings.

However, PD rights have conditions and limitations. They do not apply to:

• Flats and maisonettes
• Listed buildings
• Properties in conservation areas, Areas of Outstanding Natural Beauty, or National Parks (for some types of work)
• Properties where PD rights have been removed by a planning condition (Article 4 direction)
• Works that exceed size, height, or position thresholds

What to ask:

• "Was planning permission obtained for the extension/conversion? Can I see the approval notice?"
• "If it was done under permitted development, was a Lawful Development Certificate (LDC) obtained?" - an LDC is formal confirmation from the council that the work qualifies as PD. Without it, there's no guarantee the council would agree.
• Check the council's planning portal yourself - search by address to see what applications have been made and approved.

Building control completion certificates

Planning permission controls what you build. Building control controls how you build it. All structural alterations, including extensions and loft conversions, require building control approval under the Building Regulations. This involves:

• Submitting plans or a building notice before work starts
• Inspections at key stages during construction (foundations, structural steel, drainage, insulation, etc.)
• A completion certificate issued when the work is finished and passes final inspection

What to ask:

• "Can I see the building control completion certificate?"
• If no certificate exists: "Was a building notice or full plans application submitted?" - you can verify this with the local authority building control department.
• If neither exists, the work was done without any building control oversight. Your solicitor may obtain indemnity insurance, but this only covers the legal risk - not the physical risk of substandard work.

Why this matters for your mortgage: Most lenders require evidence that any structural alteration has building control sign-off. A missing completion certificate can delay or prevent your mortgage offer. The lender may accept indemnity insurance, but some will not - and the cost is typically £200–£500.

Structural warning signs to look for

When viewing an extended or converted property, look for physical signs of structural problems:

Extensions

• Cracking at the junction between extension and original building - a vertical crack running from ground to roof at the point where the extension meets the main house indicates differential movement. The two structures have different foundations and are settling at different rates. See our guide on spotting structural cracks.
• Sloping floors in the extension - if the floor of the extension slopes toward the garden, the foundations may be inadequate.
• Damp at the junction with the main house - poor weatherproofing where the extension roof meets the original wall is a common source of penetrating damp.
• Flat roof condition - many extensions have flat roofs with a lifespan of 10–20 years. Check for ponding, blistering, or felt that's cracking and curling.

Loft conversions

• Headroom - Building Regulations require a minimum of 2.2m headroom over at least 50% of the usable floor area. If you're ducking or can feel the ceiling close overhead, the conversion may not meet regulations - and can't legally be counted as a bedroom for valuation purposes.
• Staircase design - A proper loft conversion should have a permanent, fixed staircase - not a pull-down ladder or a steep ship's ladder. The staircase should be enclosed with a fire-rated door at the base (a 30-minute fire door, recognisable by its weight and intumescent strip).
• Fire safety - Fire doors on all rooms opening onto the staircase, mains-wired interconnected smoke alarms on every level, and an escape window in the loft room are all Building Regulations requirements for habitable loft conversions.
• Floor strength - Original loft joists are designed to support storage loads, not habitable room loads. A proper conversion requires new, deeper joists or structural steel. If the floor feels springy or bouncy, the joists may not have been upgraded.

How to assess workmanship

Beyond structural adequacy, the quality of the build affects your ongoing maintenance costs and enjoyment:

• Finish quality: Are skirting boards, architraves, and window reveals neatly finished? Do doors hang properly? Are tiles evenly laid? Poor finishing suggests the builder cut corners - and if the visible work is substandard, the hidden work may be worse.
• Integration: Does the extension or conversion feel like a natural part of the house, or does it feel bolted on? Awkward level changes, narrow corridors to reach the new space, or rooms that lose natural light due to the extension are design problems that can't easily be fixed.
• Insulation and temperature: Extensions and loft conversions should be warm in winter and cool in summer. If the extension feels noticeably colder (or hotter in a loft) than the rest of the house during your viewing, the insulation may be inadequate. This will increase energy bills and reduce comfort.
• Drainage: If the extension includes a kitchen or bathroom, check that wastewater drains properly. Slow-draining sinks can indicate poorly laid drainage - expensive to dig up and redo.

How extensions and conversions change value and risk

A well-executed extension typically adds 5–15% to a property's value, depending on the local market and the type of space added. A loft conversion adding a bedroom and bathroom is one of the highest-value alterations in UK property.

However, value is not guaranteed. Extensions that:

• Reduce the garden to an unusably small area
• Create an awkward layout (e.g., a bedroom you can only reach through another bedroom)
• Have poor natural light due to an oversized footprint or insufficient glazing
• Are visually out of keeping with the street or original house

...may actually reduce the property's value or make it harder to sell. A RICS valuer will assess the property as a whole, not the extension in isolation.

When to ask for documents or specialist input

Before making an offer on an extended or converted property, request:

• Planning permission approval notice (or Lawful Development Certificate for PD works)
• Building control completion certificate
• Structural engineer's calculations or drawings (if available)
• Building warranty documentation (if the work is less than 10 years old)
• Details of who carried out the work - a reputable builder, a specialist conversion company, or DIY

If any of these are missing and you still want to proceed, consider commissioning a Level 3 building survey with specific instructions to inspect the extension or conversion in detail. A building surveyor can assess workmanship, identify structural concerns, and flag compliance issues that your standard survey might not cover. For context on what surveys miss, see what a home survey doesn't tell you.

If the structure looks concerning - significant cracking, visibly inadequate foundations, or a poorly supported loft floor - commission a structural engineer's report (£400–£800) before proceeding. See property repair costs UK for detailed cost guidance.

Frequently asked questions

Can I check if planning permission was granted for an extension?

Yes. Most local authorities publish planning records online, searchable by address. You can see whether an application was made, what was approved, and whether conditions were attached. If no application exists and the work doesn't qualify as permitted development, it may be unauthorised - creating enforcement risk and insurance complications.

What is a building control completion certificate and why does it matter?

A completion certificate confirms building work was inspected during construction and met Building Regulations. Without it, there's no independent verification the structural design, fire safety, insulation, and drainage were done to standard. Most mortgage lenders require it for any structural alteration.

Is a loft conversion without building control sign-off a problem?

Yes, potentially. Loft conversions involve structural changes, fire safety requirements, and often electrical work - all requiring building control approval. Without sign-off, you have no assurance the work was done safely. Your solicitor can obtain indemnity insurance for the legal risk, but this doesn't address physical risk from substandard work.

Do extensions always add value?

Not always. A well-executed extension adding a bedroom or enlarging a kitchen generally adds 5–15% of property value. However, poor-quality builds, extensions that make gardens too small, or loft conversions with limited headroom can reduce value or narrow the buyer pool.

About the author

Jag Singh is a Senior Quantity Surveyor with 18 years of experience across residential and commercial property. He founded KeyWise to help UK buyers use price, condition, repair-cost and local market data to make better decisions, negotiate with confidence, and secure the right property at the right price.