The Ultimate Extension Guide for Devon & Cornwall

1. What is the extension actually for?

Before asking a builder for a price, decide what you need the new space to achieve. Two homeowners can build extensions of almost identical size but need completely different designs.

Questions worth answering before design begins

• What is frustrating you about the existing house?
• Who will use the new space?
• Does the extension need to work differently in five, ten or twenty years?
• Are you trying to create more floor area or a better layout?
• How much garden or outside space are you prepared to lose?
• How will the new room connect to the original house?
• What is your realistic total budget rather than just your construction budget?

2. Use AI to explore extension styles before committing to a design

AI image tools can now be useful during the very earliest inspiration stage. Take a clear photograph of the part of the property where the extension may be built and ask an AI image system to show indicative interpretations of different styles.

You might compare a traditional pitched-roof extension with a contemporary flat-roof design, experiment with render or brick finishes, explore larger glazed openings, compare bifold and sliding doors, or see how different window arrangements might relate visually to the existing property.

Once you have identified styles you like, those indicative images can become useful conversation starters with your architect, architectural designer, builder or planning professional.

3. Understand the property before designing the extension

Devon and Cornwall contain an unusually varied housing stock. The existing building and its site should influence the design rather than being treated as scenery around a generic extension.

If the property sits on filled ground or a modern development, read the DC Trades guide to extension foundations on filled ground and modern housing estates .

For sloping sites, our guide to planning building work at a steep South Hams property explains why access, drainage and levels should be considered early.

4. Common types of home extension

Single-storey rear extension

Often used to enlarge kitchens, dining spaces and family rooms. The design should consider how much daylight remains in the centre of the existing property once the rear wall moves outward.

Side extension

Can make productive use of an underused side passage or side garden. Boundary position, neighbouring properties, drainage and access can heavily influence feasibility.

Wraparound extension

Combines rear and side additions and can transform the ground-floor plan, but usually involves more structure, roofing, drainage and design coordination than a simple rectangular addition.

Two-storey extension

Adds accommodation at two levels and can provide strong value in terms of floor area, but structural, planning, overlooking, daylight and neighbour considerations become particularly important.

Over-garage extension

Existing foundations and garage structure need to be assessed rather than assumed suitable for the additional load.

Orangery or heavily glazed extension

Large areas of glazing can create beautiful spaces, but thermal performance, solar gain, ventilation, privacy and the relationship between glass area and orientation should be considered as part of the whole design.

5. How much does an extension cost in Devon and Cornwall?

Square-metre rates are useful for early budgeting, but they can become misleading when treated as a fixed price. The specification inside an extension can change the final cost dramatically.

For example, two extensions could have the same footprint while one contains a basic living room and the other contains a premium kitchen, utility room, bathroom, extensive glazing, bespoke joinery and upgraded heating.

Kitchens and bathrooms can materially increase the budget

A kitchen is not simply another room finish. Cabinetry, worktops, appliances, plumbing, electrics, extraction, lighting, flooring and installation all add cost. A high-end bespoke kitchen can add roughly £15,000 to £70,000 or more depending on the specification, manufacturer, appliances and level of customisation.

Bathrooms and utility rooms similarly introduce plumbing, drainage, waterproofing, sanitaryware, ventilation, tiling and additional labour.

What changes the price?

• Ground conditions and foundation design
• Site access and material handling
• Structural steelwork and size of openings into the existing house
• Roof design and roof covering
• Quantity and specification of glazing
• Windows, bifold doors, sliding doors and entrance doors
• Kitchen and bathroom specification
• Heating and hot-water alterations
• Electrical work and lighting design
• Flooring, decoration and internal finishes
• Drainage alterations
• Scaffolding and access
• Professional fees and surveys
• Planning and Building Control costs where applicable
• Unexpected defects or conditions discovered after work begins

Keep a contingency pot

Consider keeping approximately 10% of the anticipated build cost available as a contingency rather than spending every available pound on the initial specification.

A contingency is not permission for the project to drift over budget. It is protection against genuinely unforeseen issues such as hidden drainage, unexpected ground conditions, structural defects uncovered during opening-up work or necessary changes that could not reasonably have been identified beforehand.

6. Design decisions that deserve attention before building starts

Roof type

Pitched, hipped, lean-to and flat-roof designs each create different architectural, drainage, structural and maintenance considerations. The roof should relate to the existing property and the constraints of the site rather than simply following whatever was used on the previous project.

Windows and doors

Think beyond colour. Consider frame material, thermal performance, ventilation, security, opening arrangements, thresholds, sightlines and how much wall space remains for furniture.

Large bifold or sliding doors can dramatically change the relationship between a room and the garden, but larger glazed openings also affect structural design, heat loss, solar gain and privacy.

Heating

Decide how the new room will be heated before the floor and internal finishes are designed. Extending an existing radiator system, installing underfloor heating or changing the heating arrangement may have consequences elsewhere in the property.

Ventilation

Kitchens, bathrooms, utility rooms and highly insulated spaces need appropriate ventilation. Treating ventilation as an afterthought can create comfort and moisture problems later.

Electrical design

Plan socket positions, lighting circuits, appliance supplies, external lighting, data connections and charging requirements before walls are finished. Moving them later is considerably less elegant.

Drainage

Existing foul and surface-water arrangements should be understood early. Building over or close to drains, moving inspection chambers or changing large areas of hard surface can affect the design.

7. Contact an architect or architectural designer for drawings

Where drawings are required, speak to a suitably experienced architect or architectural designer before trying to obtain firm construction quotations.

They can survey the property, explore the proposed layout and prepare the drawings needed for the project and, where required, the planning application.

Clarify exactly what their appointment includes

An architectural service may include some or all of the following:

1. Measured survey of the existing property.
2. Concept design exploring the size, layout and appearance of the extension.
3. Planning drawings for submission where permission is required.
4. Building Regulations drawings or technical information.
5. Coordination with a structural engineer where calculations or structural design are needed.
6. Construction details providing greater clarity for builders and quotations.

If somebody is offering their services specifically as an architect, homeowners can also check the Architects Registration Board register.

If you are unsure whether the first professional you need is a builder or structural engineer, see our guide: Builder or structural engineer: who should you call first?

8. Planning permission, permitted development and Building Regulations

Planning permission and Building Regulations are separate systems. A project may not require a conventional planning application but can still need to comply with Building Regulations and other legal requirements.

Some extensions may fall within permitted development rights, subject to the relevant limits, conditions and the circumstances of the individual property. Rights can also be affected by matters such as previous extensions, planning conditions, Article 4 directions, listed status and location.

Because this guide covers the whole of Devon and Cornwall, homeowners should check the authority responsible for planning at the actual property rather than relying solely on a national summary.

Official planning authorities across Devon and Cornwall

Heritage buildings and conservation areas

Listed buildings, conservation areas and properties affected by Article 4 directions may have additional restrictions. The fact that a proposed extension is small does not automatically remove the need to check the relevant heritage and planning controls.

Building Regulations

Building Regulations deal with areas including structural safety, fire safety, energy performance, ventilation, drainage and other technical standards. Discuss the appropriate Building Control route for the project before construction begins.

9. Speak to your neighbours before the work begins

A short friendly conversation with neighbouring homeowners can prevent an extraordinary amount of friction later.

Something as simple as:

gives neighbours an opportunity to understand what is being proposed and to mention concerns you may not have considered, such as boundaries, access, parking, drainage or the impact of building work.

It does not give a neighbour an automatic veto over development that is otherwise lawful, but early communication can make months of deliveries, scaffolding, noise and construction considerably easier for everyone involved.

Depending on the work and its relationship with neighbouring structures or boundaries, separate requirements such as the Party Wall etc. Act may also need to be considered.

10. Foundations, structure and what is already below the ground

Foundation design should respond to the actual site and proposed building. Ground conditions, nearby trees, previous development, drainage, levels and the loads created by the extension can influence the solution.

Do not assume the foundation used on a neighbour's extension will necessarily be appropriate for yours.

Modern developments and filled ground

Some newer estates were constructed on engineered or filled ground, and some homes may themselves use specialist foundation systems. An extension should not be designed on the assumption that the visible house tells you everything about what sits underneath it.

Read: Extension foundations on filled ground and modern housing estates .

Creating large openings

Opening the original rear or side wall to connect a new extension can require structural design and steelwork. This needs to be coordinated with the layout, roof loads, support conditions and Building Regulations rather than treated as a decorative alteration.

11. Choosing the right builder or extension company

A beautiful set of drawings can still become a poor project if the contractor is chosen solely on the lowest number at the bottom of a quotation.

Compare at least three reputable firms

Where practical, obtain quotations from at least three reputable building firms. The point is not simply to find the cheapest quote. It is to compare what each contractor believes they are actually pricing.

One quotation may include waste removal, scaffolding, decoration and Building Control coordination while another quietly excludes them. A lower headline number is not cheaper if significant parts of the project have simply been left outside the price.

DC Trades has a dedicated guide to comparing building quotes with different specifications .

Questions to ask the builder

• What exactly is included in the quotation?
• What is specifically excluded?
• Is VAT included where applicable?
• Who will supervise the project?
• Who will actually carry out the work?
• Do you use your own workforce or subcontractors?
• Who arranges structural engineering?
• Who liaises with Building Control?
• Are scaffolding and access equipment included?
• Is waste removal included?
• Who supplies windows and doors?
• Who supplies the kitchen or bathroom?
• How are changes to the specification priced?
• What is the expected start date?
• What is the estimated build programme?
• What happens if work is delayed?
• What insurance does the business hold?
• Can you provide evidence of similar completed projects?
• What guarantees apply to the workmanship?
• Which product warranties will be supplied?
• What documentation will I receive at completion?

Check the financial health of the company

Homeowners cannot normally inspect a contractor's current bank balance, and a single financial figure would not tell the whole story anyway. What you can do is look for evidence that the company is established and trading responsibly.

Where the contractor is a limited company, review its filing history at Companies House, look for overdue filings or other warning signs, understand how long it has been trading, check relevant insurance and be particularly cautious where very large upfront payments are requested without a clear commercial reason.

Read the DC Trades guide: What Companies House records can and cannot tell you about a tradesperson .

12. Agree the payment timetable before construction starts

Payment arrangements should be discussed and written down before the project begins. Avoid reaching the middle of a major build with homeowner and contractor holding different assumptions about when money is due.

Depending on the project and contract, payments may be linked to meaningful stages of work rather than arbitrary calendar dates. The important point is that both sides understand the arrangement before work starts.

DC Trades covers this subject in more depth in Deposits, staged payments and variations in building work .

Control variations

Extensions evolve. A homeowner may change a kitchen, add electrical work or alter a finish after the original quotation. Unexpected site conditions can also require additional work.

Variations should be documented and priced as clearly as possible rather than allowed to accumulate into a mysterious figure at the end of the project.

Product warranty and fitting guarantee are not the same thing

This distinction matters particularly with products such as windows, doors, roof systems, boilers, appliances and specialist finishes.

A manufacturer's product warranty may cover faults with the product under particular terms. The contractor's workmanship or fitting guarantee relates to the installation. The duration, exclusions and provider can be different.

13. Think about when the extension will be built

Devon and Cornwall do not provide builders with a guaranteed dry season. However, the time of year and likely weather conditions can affect the sequencing and programme of a project.

Prolonged rain or high winds can affect excavation, masonry, roofing, scaffolding, external finishes, deliveries and the periods during which the existing house is opened to the new extension.

This does not mean extensions should only be built in summer. Professional builders work throughout the year. It does mean that homeowners should discuss weather protection, programme allowances and how the house will be kept secure and weather-tight during key stages.

Living in the house while the extension is built

This deserves planning of its own, especially for larger kitchen and family-room extensions.

• Where will you prepare food when the existing kitchen is disconnected?
• Will water or electricity need to be temporarily interrupted?
• Can children safely reach the garden?
• How will pets be separated from the work area?
• Where will building materials be stored?
• Where will skips and delivery vehicles go?
• At what stage will the original house be opened into the extension?
• How will dust be controlled?
• Can the property remain secure overnight?
• Is temporary accommodation worth considering during the most disruptive stage?

14. Plan the end of the project before you reach it

Completion should mean more than the builders packing their tools away.

Before final handover, establish what documentation should be supplied

• Building Control completion documentation where applicable
• Electrical certification where applicable
• Gas documentation where applicable
• Window and door documentation
• Manufacturer warranties
• Installer or workmanship guarantees
• Heating-system information
• Appliance manuals and warranties
• Details of any specialist systems or finishes
• Final variation and payment record

Snagging

Inspect the completed work while the project history is still fresh. Small defects are far easier to identify, record and resolve when everyone understands exactly what was included in the original project.

15. A sensible extension journey from first idea to completion

1. Define what you want the extension to achieve.
2. Set a realistic overall budget and contingency.
3. Explore design ideas, including indicative AI visualisations if useful.
4. Understand the property, site, drainage, access and likely constraints.
5. Appoint an architect or architectural designer where appropriate.
6. Check planning, heritage and permitted-development requirements.
7. Obtain structural and technical input where required.
8. Establish the Building Control route.
9. Develop enough specification detail for meaningful quotations.
10. Compare at least three reputable contractors where practical.
11. Carry out due diligence before appointing the builder.
12. Agree the contract, payment timetable and variation process.
13. Plan how the household will operate during construction.
14. Monitor changes and keep important decisions in writing.
15. Complete snagging and collect certificates, warranties and guarantees.

📍 Find Extension Builders & Specialists Near You

Explore local extension services and verified building companies across Devon & Cornwall:

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