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What changes the cost of a flat roof extension?
In Cheshire, the cost of a flat roof extension depends on its size, the foundations and ground, the roof structure and covering, the glazing, the kitchen or bathroom inside, and the design, planning and building control work around it. We print no build price, so compare written quotes priced from the same drawings.
Last reviewed:What does a flat roof extension price include?
A flat roof extension is a new room, usually single storey and usually at the back or side of the house, with a roof that has only a slight fall for drainage. The price is a price for a whole building project. It typically covers the design, planning or a permitted development check, building control, groundworks and foundations, walls, the roof structure and covering, windows, doors and roof lights, opening into the existing house, drainage, electrics, heating, plastering and finishes. A kitchen or bathroom inside is often a separate line.
QuoteZila is starting in Cheshire, with Crewe, Nantwich and Sandbach among the first towns. This guide does not print an extension price. We could not find two dated public UK sources for one, and a figure that depends so much on the specification is not something a single article can settle. What this guide covers is what changes the cost, what the rules say, and how to make quotes comparable.
Planning permission and permitted development
Many single-storey rear extensions can be built as permitted development, without a full planning application, if they meet the limits. The Planning Portal says that, without prior approval, a single-storey rear extension can extend up to four metres beyond the rear wall of the original house if it is detached, or three metres for any other house [S3]. It also says single-storey rear extensions cannot exceed four metres in height, and that where the extension comes within two metres of the boundary, the height at the eaves cannot exceed three metres [S3].
The same guidance says these rights apply to houses, not flats or maisonettes, and that a planning condition, an Article 4 direction or another restriction can limit or remove them [S3]. Listed buildings and conservation areas bring their own rules. If you are close to the limits, ask the council, or apply for a lawful development certificate before building.
A flat roof is often chosen partly because it keeps the height down near the boundary. Ask the designer how the roof height and the eaves line relate to those limits.
Building regulations and structural design
Planning and building regulations are separate. The Planning Portal says most extensions of a property require approval under the building regulations [S4]. That covers the foundations, structure, insulation, ventilation, drainage, fire safety and electrics. Most projects need drawings and, for steel beams and the opening into the house, structural calculations.
Ask each builder:
- whether they are pricing from approved drawings, or from a sketch
- who will make the building control application and arrange inspections
- whether building control fees, the engineer and the drawings are in their price
- what certificate you will receive at the end
This page gives no DIY steps for structural work.
GOV.UK: find your local council for building control. QuoteZila is not affiliated with this organisation.
What changes the cost of a flat roof extension
| What is being priced | What makes it bigger or smaller | What to ask |
|---|---|---|
| Design and drawings | Architect or designer, structural engineer | Whether design fees are in the price or separate |
| Planning | Permitted development, prior approval or full application | Which route applies, and who handles it |
| Building control | Application and inspections | Who applies, and whether fees are included |
| Size and shape | Floor area, depth, width, corners | The dimensions they priced from |
| Ground and foundations | Clay, trees, drains, slopes, soft ground | Foundation type assumed, and how extras are priced |
| Walls | Brick, block and render, cladding | The wall build-up and finish |
| Roof structure and covering | Joists, insulation, deck, membrane or GRP | The covering named, and who installs it |
| Roof lights and lanterns | Number and size of roof lights | What is included and the make |
| Doors and windows | Bifold or sliding doors, windows | The make, size and glazing |
| Opening into the house | Steel beam, temporary support | Whether the steel and support are in the price |
| Drainage | New gutters and downpipes, moving drains | What drainage work is included |
| Electrics and heating | Lighting, sockets, radiators, underfloor heating | Who does them, and how electrics are certified |
| Inside finish | Plaster, floor screed, flooring, decoration | Where their price stops |
| VAT | Whether the business is VAT registered | Whether the total includes VAT |
A business must register for VAT when its taxable turnover goes over £90,000 [S2]. On a project this size, the VAT line makes a large difference, so ask whether each total includes it.
The roof itself
The roof is where flat roof extensions earn or lose their reputation. A good flat roof has a deliberate fall so water runs off, enough insulation to meet the regulations, ventilation or a warm roof build-up to manage condensation, careful detailing where it meets the house wall, and proper outlets and gutters.
Ask each builder:
- what covering they are quoting, such as a single-ply membrane, glass reinforced plastic or a built-up felt system
- whether the roof is a warm roof or a cold roof build-up, and what insulation is used
- who installs the covering, and whether that installer gives a separate warranty
- how the roof meets the existing wall, and what flashing is used
- how roof lights or a lantern are sealed into the roof
If an existing flat roof on the house is being re-covered at the same time, the Planning Portal says you will need approval if a new covering increases the weight of the roof covering by 15 per cent or more, or if structural alterations are made [S5].
Working at height and site safety
Even single-storey work involves the roof edge, and joining a new roof to the house can mean working near upper windows and the main roof. The Health and Safety Executive's order of approach is to avoid work at height where it is reasonably practicable, then prevent falls using an existing safe place or the right type of equipment, and then minimise the distance and consequences of a fall [S6]. Ask the builder what scaffolding or edge protection they will use, and whether it is in the price.
What to have ready before you ask for quotes
Builders can only price what they can see. Give each one:
- drawings, if you have them, or a sketch with rough dimensions
- what the room is for: kitchen, living space, bathroom, utility
- the doors, windows and roof lights you want
- photos of the back of the house and the garden, including manhole covers, trees and boundaries
- whether you share a wall or a boundary with a neighbour close to the work
- whether planning permission or a lawful development certificate is already in place
- your postcode, and when you would like to start
If you do not have drawings yet, some builders will quote for a design-and-build package. That is a different kind of quote from one priced against an architect's approved drawings, so keep the two apart when you compare.
How to compare flat roof extension quotes
Check each quote for:
- The same drawings, or the same dimensions and specification.
- Who handles planning, building control and the engineer, and whether those fees are included.
- The foundation type assumed, and how unexpected ground conditions are priced.
- The roof build-up and covering, named.
- Doors, windows and roof lights, named.
- Electrics, heating and drainage.
- Where the price stops inside the room.
- A payment schedule tied to stages of work.
- Whether VAT is included.
Citizens Advice suggests getting written quotes from at least three traders, avoiding large upfront payments and keeping any deposit you cannot avoid to no more than 25% [S1].
Jobs you might post
These are examples, not taken from real job posts. "Build a single-storey flat roof extension at the back of a semi-detached house for a larger kitchen. We have approved drawings." Or: "Quote for a side extension with a flat roof and a roof lantern, design included, on a house in Sandbach." Or: "We need a builder to price a small rear extension for a utility room and downstairs toilet."
Get quotes from local builders
Describe the extension, the drawings you have and your postcode. Up to five matched local businesses can reply [QZ2]. QuoteZila is free for customers and local businesses [QZ1].
Honest status: QuoteZila is new. We're starting in Cheshire, beginning with Crewe, Nantwich and Sandbach, and growing area by area. Anyone in the UK can still post a job or register a business. How fast you get quotes depends on how many local businesses have joined near you. Businesses can register and receive jobs straight away. We don't check insurance, qualifications or registrations, so please check these yourself before you hire.
Posting a job is not open yet
See the builders page, the roofers page or the electricians page. For a solid roof on an existing conservatory instead, see tiled conservatory roof cost.
Official and public sources
QuoteZila is not affiliated with these organisations.
- Quotes and deposits: Citizens Advice [S1].
- VAT: GOV.UK on when to register [S2].
- Planning: Planning Portal: extensions, planning permission [S3].
- Building regulations: Planning Portal: is building regulations approval needed for an extension [S4].
- Existing roofs: Planning Portal: work to an existing roof [S5].
- Work at height: HSE introduction to working at height safely [S6].
Questions about flat roof extension costs
How much does a flat roof extension cost?
This guide does not print a figure. We could not find two dated public UK sources for an extension build price, and a single number from one survey is not enough to print. Size, ground conditions, foundations, the roof, the glazing and the fit-out all change the total. Ask builders to price the same drawings.
Do I need planning permission for a flat roof extension?
Not always. The Planning Portal says a single-storey rear extension can be permitted development within limits, including a depth of up to four metres for a detached house or three metres for other houses, and a height of up to four metres. Other conditions and local restrictions apply, so check with your council.
Do I need building regulations approval for an extension?
Almost always. The Planning Portal says most extensions of a property require approval under the building regulations. Ask each builder who will make the application and arrange the inspections, and whether those fees are in the price.
Should an extension have a flat roof or a pitched roof?
A flat roof can be simpler to build, but this guide does not compare prices. The choice also depends on the look you want, the height limits near the boundary and the drainage. Ask each builder to price both options from the same plan if you are undecided.
What flat roof covering should I ask about?
Common options include single-ply membranes, glass reinforced plastic and built-up felt systems. This page does not rank them. Ask each builder which covering they are quoting, who will install it, how long the maker's warranty is and what it covers.
Should I pay a deposit for an extension?
Citizens Advice suggests avoiding large upfront payments and keeping any deposit you cannot avoid to no more than 25%. For a larger build, agree stage payments in writing that match finished stages of work.
Does QuoteZila check the builders who quote?
No. We don't check insurance, qualifications or registrations, so please check these yourself before you hire. You can report a quote or a business from your job page.
Related guides
- Builders: extensions and structural work
- Roofers: flat roof coverings and repairs
- Roof repair cost: another guide to reading a roofing price
- All cost guides
Part of Cost guides. All services
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Related pages
- Floor Screed Cost: What Changes the Price. Screed type, area, depth, underfloor heating, insulation and drying: what moves a floor screed quote in Cheshire, and what each written quote should list.
- Garage Conversion Cost in Cheshire: What to Ask. A Cheshire garage converted into a room is often internal work, but a habitable room needs building regulations approval. Ask what the existing shell needs.
Sources
- QZ1: QuoteZila product fact. QuoteZila home and jobs-for-tradesmen copy.
- QZ2: QuoteZila product fact. QuoteZila terms and matching notes.
- S1: Before you get work done on your home, Citizens Advice. Accessed 2026-10-25. https://www.citizensadvice.org.uk/consumer/getting-home-improvements-done/before-you-get-building-work-done/
- S2: Register for VAT, GOV.UK. Accessed 2026-10-25. https://www.gov.uk/register-for-vat
- S3: Planning permission: extensions, Planning Portal. Accessed 2026-10-25. https://www.planningportal.co.uk/permission/common-projects/extensions/planning-permission/
- S4: Is building regulations approval needed for an extension?, Planning Portal. Accessed 2026-10-25. https://www.planningportal.co.uk/permission/common-projects/extensions/is-building-regulations-approval-needed-for-an-extension/
- S5: Building regulations: work to an existing roof, Planning Portal. Accessed 2026-10-25. https://www.planningportal.co.uk/permission/common-projects/roof/building-regulations-work-to-an-existing-roof/
- S6: Introduction to working at height safely, Health and Safety Executive. Accessed 2026-10-25. https://www.hse.gov.uk/work-at-height/introduction.htm