Selling a House with an Extension: What You Need

Extensions are one of the most popular home improvements in the UK. Whether you added a kitchen extension, a rear return, a side extension, or built above an existing structure, the work should have been carried out with the proper approvals in place. When it comes time to sell, you need to prove this to the buyer, their solicitor, their mortgage lender, and their surveyor.

This guide explains exactly what documentation you need when selling a house with an extension, what happens if paperwork is missing, and how to prepare so that the extension helps rather than hinders your sale. It covers planning permission, building regulations,permitted development, indemnity insurance, and how extensions affect your property's value.

Why extensions create extra work when selling

When a buyer's solicitor reviews the title and property information for a house with an extension, they are looking for answers to two fundamental questions:

1. Was the extension lawfully built? This means confirming that planning permission was obtained (or that the work fell within permitted development rights) and that building regulations were complied with.
2. Is the extension structurally sound and safe? This is assessed through the buyer's survey, but documentary evidence of building regulations compliance provides additional reassurance to both the buyer and their mortgage lender.

If you can provide clear documentary answers to both questions, the extension should not slow down your sale. Problems arise when documentation is incomplete or missing entirely, which is surprisingly common — particularly for extensions built more than 10 to 15 years ago or carried out by previous owners.

Planning permission: what you need to show

Every extension requires planning consent of some kind. In practice, this means either full planning permission from the local planning authority or confirmation that the work fell within permitted development rights under the Town and Country Planning (General Permitted Development) (England) Order 2015 (as amended).

If planning permission was granted

If you or a previous owner applied for and received full planning permission for the extension, you need:

• The decision notice (planning approval letter) from the local planning authority
• The approved plans and drawings that were submitted with the application
• Evidence that any planning conditions were discharged (some permissions are granted subject to conditions that must be met before, during, or after construction)

If you have lost these documents, you can usually obtain copies from the local planning authority. Most councils maintain online planning portals where decisions from the last 10 to 20 years can be viewed and downloaded free of charge. For older applications, a written request to the planning department may be necessary.

If the extension was built under permitted development

Many extensions — particularly single-storey rear extensions — can be built without a full planning application under permitted development rights. These rights allow homeowners to carry out certain types of work provided the extension stays within specified size limits, height restrictions, and other conditions.

The difficulty for sellers is that permitted development does not generate a planning approval document by default. There is no automatic certificate confirming the work was within the allowed limits. To provide proof, you have two options:

• Lawful development certificate (LDC). This is a formal certificate issued by the local planning authority confirming that the development is lawful. You can apply for a certificate of lawful existing use or development (CLEUD) if the work is already complete, or a certificate of lawful proposed use or development (CLOPUD) before starting work. The application fee is £103 (half the standard planning application fee), and processing typically takes six to eight weeks. This is the gold standard of evidence for permitted development work.
• Building control records and measurements. If you do not have an LDC, your solicitor can demonstrate compliance through building control records, approved plans showing dimensions, and photographs. However, this is less definitive and may generate additional buyer enquiries.

Permitted development limits for extensions

The following table summarises the key permitted development limits for extensions to houses in England. These limits apply to houses that have not been previously extended beyond the original dwelling and are not in designated areas such as conservation areas, Areas of Outstanding Natural Beauty, or national parks.

If your extension exceeds any of these limits, it required full planning permission. If it was built without planning permission and is less than four years old, the local authority can take enforcement action requiring you to alter or demolish the work. After four years, the development becomes immune from enforcement under Section 171B of the Town and Country Planning Act 1990, and you can apply for a CLEUD to confirm its lawful status.

Building regulations: the completion certificate

Separately from planning permission, virtually all extensions must comply with the Building Regulations 2010 (as amended). Building regulations cover structural integrity, fire safety, energy efficiency, drainage, ventilation, and electrical safety. When the work is complete and passes a final inspection, the local authority or an approved inspector issues a building regulations completion certificate.

This certificate is one of the most important documents in any house sale involving an extension. The buyer's solicitor will request it as standard, and the buyer's mortgage lender will expect to see it (or an acceptable alternative such as indemnity insurance). For a detailed explanation of what happens when this certificate is missing, see our guide on selling with no building regulations certificate.

What to do if the completion certificate is missing

Missing building regulations completion certificates are extremely common, particularly for older extensions. The previous owner may never have applied for building control sign-off, the builder may not have requested the final inspection, or the certificate may simply have been lost over the years. You have several options:

• Contact the local authority. If building control was involved at the time of construction, the council may have records and may be able to issue a copy of the completion certificate. However, if the work was never inspected, this route will not be available.
• Regularisation. Under Section 36 of the Building Act 1984, a local authority can retrospectively inspect and approve building work that was carried out without building regulations approval. This involves a building control surveyor visiting the property, inspecting what they can see, and issuing a regularisation certificate. The fee varies by council but is typically £400 to £1,500 depending on the scope of work. The downside is that the inspector cannot see hidden elements (foundations, cavity insulation, damp-proof membranes), so regularisation may not always be straightforward.
• Indemnity insurance. This is the most common solution when a completion certificate is missing. An indemnity insurance policy covers the buyer and their lender against the financial risk of the local authority requiring alterations to or removal of the extension. Policies are arranged by the seller's solicitor and cost a one-off premium of £100 to £300. Most mortgage lenders accept this as an alternative, provided the work was completed more than 12 months ago.

Important: if you intend to rely on indemnity insurance, you must not contact the local authority about the building work. Any contact with the council about the extension can invalidate the insurance, because the policy is predicated on the assumption that the authority is unaware of the non-compliance. Always speak to your solicitor before making any enquiries with the council.

Documents checklist for selling with an extension

The following table summarises the documents you should gather before listing your property. For a broader overview of sale documents, see our guide on documents needed to sell a house.

How extensions affect property valuations

A well-executed extension with full documentation generally adds significant value to a property. However, the amount varies depending on the type of extension, the quality of the build, and the local market. The following estimates are based on industry data and RICS guidance:

It is worth noting that extensions without proper documentation can reduce rather than add value. Buyers will factor in the risk and cost of resolving missing paperwork, and some may reduce their offer accordingly or withdraw altogether. The surveyor's report will highlight any concerns about the physical quality of the extension and the completeness of the supporting documentation.

What buyers and surveyors look for

Understanding what the buyer's side will scrutinise helps you prepare effectively. Here is what each party typically examines:

The buyer's solicitor

• Evidence of planning permission or permitted development compliance (decision notice or lawful development certificate)
• Building regulations completion certificate or an acceptable alternative (regularisation certificate or indemnity insurance)
• Consistency between the approved plans and what was actually built
• Your answers on the TA6 form regarding building works and alterations (Section 6)
• Whether any guarantees or warranties are in place for the work

The surveyor

• Structural integrity. Signs of movement, cracking, or settlement where the extension meets the original building
• Roof construction. Whether the extension roof is properly built, weathertight, and adequately drained
• Damp and drainage. Adequate damp-proof courses, proper drainage falls, and no signs of water ingress
• Quality of finish. General standard of workmanship, including plastering, flooring, and integration with the original house
• Compliance indicators. Whether the extension appears to have been built to a professional standard consistent with building regulations requirements

The mortgage lender

• Confirmation that the extension has building regulations approval (or indemnity insurance as an alternative)
• That the extension does not negatively affect the surveyor's valuation or the property's mortgageability
• That the work was carried out lawfully and will not give rise to enforcement action by the local authority

Common enquiries about extensions during conveyancing

The buyer's solicitor will raise pre-contract enquiries about your extension as part of the standard conveyancing process. Being prepared for these enquiries and providing comprehensive answers upfront is one of the best ways to keep your sale on track. Common enquiries include:

1. When was the extension built? The date of construction is important for assessing enforcement risk and determining which regulations applied at the time.
2. Was planning permission obtained? Provide the decision notice and application reference number, or confirm permitted development compliance with a lawful development certificate.
3. Was building regulations approval obtained? Provide the completion certificate, or confirm that indemnity insurance is being arranged.
4. Who carried out the work? Provide the name of the builder and, if applicable, the architect or structural engineer involved.
5. Are there any guarantees or warranties? Insurance-backed guarantees or similar protections should be provided if available. These are transferable to the new owner in most cases.
6. Was a party wall notice served? If the extension was built on or near the boundary with a neighbouring property, the buyer's solicitor will ask whether the requirements of the Party Wall etc. Act 1996 were followed.
7. Does the extension match the approved plans? If any variations were made during construction, the buyer's solicitor will want to know whether amended approval was obtained.

Preparing answers and supporting documents for these enquiries before you list — ideally as part of your upfront sale preparation — is the most effective way to prevent delays. This is the approach Pine supports: getting your sale documents prepared before marketing so your solicitor can send a completedraft contract pack to the buyer's side as soon as an offer is accepted.

Indemnity insurance for extensions: how it works

Where building regulations or planning paperwork is missing and cannot be easily obtained, indemnity insurance offers a practical solution. The policy is a one-off purchase (not an annual renewal) and covers the financial loss the buyer or their lender would suffer if the local authority were to take enforcement action — for example, requiring the removal or alteration of the extension.

Key points about indemnity insurance for extensions:

• The policy is arranged by the seller's solicitor and the seller typically pays the premium
• Cover lasts indefinitely and passes automatically to future owners without additional payment
• Most mortgage lenders accept indemnity insurance as an alternative to building regulations sign-off, provided the work was completed more than 12 months ago
• The policy is invalidated if anyone contacts the local authority about the missing approvals — always speak to your solicitor before making enquiries with the council
• Premiums typically range from £100 to £300 depending on the property value and the nature of the missing approval

Specific extension types: additional considerations

Loft conversions

Loft conversions are subject to additional requirements around fire safety (protected escape routes, fire doors, smoke detectors) and structural loading. They require building regulations approval in all cases. If a dormer window was added, planning permission may also have been needed depending on the size and position. See our full guide on selling a house with a loft conversion for specific documentation requirements.

Garage conversions

Converting a garage to living space does not usually require planning permission (unless you are changing the use of the building significantly or live in a conservation area), but it does require building regulations approval for insulation, ventilation, fire safety, and structural work. See our guide on selling a house with a garage conversion for the full list of documents needed.

Conservatories

Conservatories have specific rules around building regulations exemptions. Those built before 1 October 2008 were largely exempt. After that date, building regulations apply if the conservatory exceeds 30 square metres, has fixed heating connected to the central heating system, or is not thermally separated from the main house by external-quality doors and windows. If a conservatory has been fully integrated into the house (for example, the separating wall has been removed), building regulations apply regardless of the date of construction.

Permitted development: common pitfalls for sellers

Many sellers assume their extension was permitted development but cannot prove it. This creates difficulties during conveyancing because the buyer's solicitor cannot simply take the seller's word for it. Common pitfalls include:

• Exceeding the cumulative size limits. Permitted development limits are calculated based on the original house (as it existed on 1 July 1948 or when it was first built, if later). If the house already had an extension when you bought it, the combined total of all extensions must still fall within the permitted development limits.
• Designated area restrictions. Properties in conservation areas, national parks, Areas of Outstanding Natural Beauty, or the Broads have more limited permitted development rights. Some Article 4 directions remove permitted development rights entirely for certain types of work.
• Prior notification requirements. Larger home extensions (those between the standard limits and the extended limits) require a prior notification application to the council, giving neighbours a chance to comment. If this was not done, the extension may not benefit from the larger permitted development limits.
• Flats and maisonettes. Permitted development rights for extensions apply only to houses. If you own a flat or maisonette, permitted development does not cover extensions.

How to prepare your extension for sale

Follow these steps to ensure your extension documentation is in order before you list:

1. Gather all documentation. Locate your planning permission decision notice, building regulations completion certificate, approved plans, structural engineer's calculations, and any guarantees or warranties. If the extension was built by a previous owner, check the documents you received when you purchased the property.
2. Check for gaps early. If you cannot find the completion certificate, contact the local authority building control department to see if they hold records. Do this only if you believe building control was involved at the time — if the work was never inspected, contacting the council could invalidate the indemnity insurance option.
3. Instruct your solicitor early. Provide the extension documents as soon as you instruct them. They can identify any gaps and advise on the best way to resolve them before a buyer is found.
4. Complete the TA6 accurately. Section 6 of theTA6 Property Information Form covers building works, and you must declare the extension and the approvals obtained. Inaccurate or incomplete answers will generate enquiries and delays. For the full list of documents to prepare, see our guide on documents needed to sell a house.
5. Update your buildings insurance. Make sure your insurance policy reflects the increased floor area and rebuild cost following the extension. The buyer's solicitor may ask for confirmation of adequate cover.

Extensions compared with other common alterations

Extensions share many documentation requirements with other types of building work. If your property also has a loft conversion or garage conversion, you will need the same types of evidence for each alteration:

Sources

• Planning Portal — permitted development rights for householders (planningportal.co.uk)
• GOV.UK — Town and Country Planning (General Permitted Development) (England) Order 2015 (legislation.gov.uk)
• GOV.UK — Building Act 1984, Section 36 (regularisation of building work) (legislation.gov.uk)
• HM Government — The Building Regulations 2010 (legislation.gov.uk)
• RICS — Home surveys and the impact of extensions on property value (rics.org)
• LABC — Local Authority Building Control guidance on completion certificates (labc.co.uk)
• Law Society — Property Information Form (TA6), 4th edition, 2020 (lawsociety.org.uk)
• Law Society — Conveyancing Protocol, 5th edition (lawsociety.org.uk)
• Party Wall etc. Act 1996 (legislation.gov.uk)
• GOV.UK — Apply for a lawful development certificate (gov.uk/guidance/lawful-development-certificates)

Related guides

• Selling a House with an Annexe: Planning and Tax Issues
• Selling a Converted Barn
• Selling a Converted Commercial Property
• Selling a House with a Self-Contained Flat

Frequently asked questions