Cost guide
House extension costs in Kent — scope, specification and what a quotation must show
Homeowners researching an extension in Medway and surrounding Kent need more than a single internet rate. This guide explains structural shell versus shell-to-plaster, essential through premium turnkey packages, what is usually included or excluded, and why Napwood still quotes after seeing the house.
Reviewed 2026-10-27. Rules and fees change, so check the sources before you rely on a figure.
Why there is no single extension price
There is still no single price for a house extension in Kent. Two houses with the same floor area in Rainham and Rochester can diverge once ground conditions, steels, drainage, glazing and the kitchen are priced.
Napwood does not publish a £ per square metre rate card or anonymised contract totals on this site. The number for your house is the survey quotation against a named specification. Use this page to understand scope language, statutory fees you can check yourself, and the cost drivers that sit outside a simple floor-area guess.
Structural shell versus shell-to-plaster
These definitions are Napwood’s working language for comparing scope. Other firms may use the same words differently — always read the schedule of works.
Structural shell usually covers foundations, walls, roof structure and covering to a weatherproof stage, structural openings as drawn, and basic external doors or temporary weatherproofing where agreed. Services are typically first-fix only if included at all. Internal finishes, second-fix electrics, heating, flooring, decoration and kitchen are normally out.
Shell-to-plaster adds the work to bring the envelope to a plastered (or boarded and skimmed) interior: windows and external doors as specified, insulation to the agreed standard, first-fix and much of the second-fix wiring and plumbing preparation, plastered walls and ceilings. Floor finishes, decoration, sanitaryware, kitchen units and appliances are commonly still out unless listed.
- Ask where weatherproofing stops and internal finishes begin
- Confirm whether windows and external doors are in the shell price
- Do not compare a shell quotation with a turnkey quotation on price alone
Essential, quality and premium turnkey
Essential turnkey aims at a complete, usable room to a practical family specification: finishes, decoration, flooring of an agreed grade, heating and electrics complete, and ironmongery. Kitchen and utility packages, if included, are at an essential commercial grade defined in the quotation. Feature glazing, bespoke joinery and high-end appliance packs are usually upgrades.
Quality turnkey raises the agreed specification on windows and doors, roofing details, flooring, sanitary or kitchen grade, and making-good against the existing house. This is the level many Medway family kitchen-diners aim for when drawings and samples are fixed early.
Premium turnkey covers bespoke or high-performance fabric, larger or more complex glazed openings, refined internal joinery, and finishes chosen as a designed set rather than a builder’s standard range. Because the specification drives the price, premium work is open-ended until samples and drawings are locked.
- Match samples and drawings to the specification named in the quote
- A turnkey price without kitchen is not the same as turnkey with kitchen
- Ask which bifold or sliding door system is allowed in the package
What is usually included and excluded
Unless a quotation line says otherwise, treat the following as outside a build-only package. The quotation must state which items are in contract.
- Measured survey, architectural drawings and revisions
- Structural engineer calculations and site inspections
- Planning, prior approval or lawful development certificate fees
- Building control fees
- Party wall surveyor(s) and any award
- Asbestos survey or removal if required
- Kitchen supply and appliances unless a defined package is included
- Furniture, soft furnishings and landscaping beyond making-good the dig zone
- Unexpected ground improvement after dig
- Client-driven design changes after the price is agreed
Ground conditions, drainage and structural complexity
Ground conditions, drainage diversion, steel spans, roof type, glazing, services capacity, making-good into the existing house, and access for materials often decide the real number more than floor area alone.
A compact side return can look straightforward until you add a structural opening and a kitchen. Difficult access, deep foundations, large openings, complex roofs or high-spec glazing all change labour, temporary works and programme. Compare quotations on the same drawings, the same glazing and the same answer to who supplies the kitchen.
- Foundations and ground conditions
- Drainage routes and diversions
- Structural steelwork
- Glazing and roofing systems
- Access, scaffold and temporary works
Planning permission and Building Regulations
Cost planning and legal compliance are linked. The wrong assumption about the planning route wastes fees and redraws.
Many rear extensions fall under permitted development or the larger home extension prior-approval route; others need a householder planning application. Conservation areas, listed buildings, Article 4 directions and flats change the answer.
From 1 April 2026, the Planning Portal fee schedule for England lists £548 for a householder application to enlarge or alter a single dwellinghouse, and £249 for prior approval of a larger single-storey rear extension. A proposed lawful development certificate is half the relevant full application fee. Confirm fees on the Planning Portal calculator in the month you apply.
Medway offers optional householder pre-application advice — confirm the current meeting fee on the council site before you rely on an older figure. Swale advice charges for Sittingbourne can differ even though the national application fee is shared.
Almost every extension needs Building Regulations compliance. Fees depend on the drawings. In Medway, Gravesham, Swale and Canterbury, building control is commonly handled by the STG Building Control Partnership; a registered building control approver is an alternative. Completion certification matters when you sell.
Architect and engineer fees, VAT and contingency
Planning and building-control fees, measured surveys, architectural drawings and structural calculations usually sit outside a build-only figure. Architect and engineer appointments are their own quotations.
For most extensions and improvements to a house you live in, a VAT-registered builder charges the standard rate of 20 per cent. A reduced rate can apply to qualifying work on a dwelling empty for at least two years, and zero-rating can apply to a genuinely new self-contained house. An annexe that cannot be sold separately is not that kind of new dwelling. The quotation should say whether the total includes VAT.
A contingency in the order of around 10 per cent of the build is a common planning allowance on domestic extensions; more is wise on older houses where little is known about foundations and drainage. Contingency should be a named line, not a hidden pad on every rate. It is not a licence to upgrade finishes mid-project without resetting the budget.
Kitchen, internal fit-out and provisional sums
Kitchen supply is one of the largest swing factors between quotes. The contract must say whether units, worktops, appliances and fitting sit inside the build package or are client-supplied. Internal fit-out — flooring grade, decoration, sanitaryware, ironmongery and making-good into existing rooms — should be specified the same way when you compare builders.
Provisional sums cover items that cannot be firmly priced until design or site information is clearer (for example specialist drainage connections or finish selections still open). A provisional sum is not a free upgrade pot: when the real cost is known, the contract sum is adjusted against that line. Ask which items are firm, which are provisional, and what assumption sits behind each provisional sum.
- Kitchen in or out of contract, stated clearly
- Flooring, decoration and sanitary grades named
- Provisional sums listed with their assumptions
- Hold scope still while you compare builders
How Napwood estimates and quotes
We start with a brief: the rooms you need, whether the kitchen is in the contract, and any planning constraints you already know. We survey the property, write a clear scope with inclusions and exclusions, and issue a staged quotation against that scope with VAT stated clearly.
Design fees and council fees stay visible as separate items when they are not part of our contract. Payments follow milestones on site. Eligible Napwood work carries a 12-month workmanship guarantee, subject to written terms.
We will not give a build price from a web form alone. If you already have drawings, send them with the enquiry so every builder is pricing the same scheme.
Call 07591 484 863 or 07828 333 390 if you want a first conversation about an extension in Medway, Sittingbourne or the surrounding North and Mid Kent area.
Statutory fees you can still check yourself
The pound figures below are statutory or published council fees. They are not the cost of building the extension. Confirm each fee on the issuing site before you rely on it.
| Item | What it covers | Figure in this guide |
|---|---|---|
| Householder planning application | National fee if a full application is needed to alter one house | £548 from 1 April 2026 |
| Larger single-storey rear extension | Prior approval where the larger home extension route applies | £249 |
| Proposed lawful development certificate | Formal confirmation that the work is lawful | Half the relevant application fee |
| Medway pre-application meeting | Optional advice from Medway planning officers | Confirm current Medway fee list |
| VAT on the build | Most work to an existing occupied house by a VAT-registered builder | Standard rate, 20 per cent |
| Drawings, engineer, party wall, building control | Third-party appointments | Their own quotations |
| Build, glazing and kitchen | The construction contract | Napwood survey quotation against a named specification |
FAQs
Frequently asked questions
Do you publish a £ per square metre build rate?
No. This guide explains specification levels and cost drivers. Your quotation follows a survey and a defined scope.
What is the difference between shell and turnkey?
Shell stops at a weatherproof structure (or shell-to-plaster if that package is priced). Turnkey aims at a finished, usable room to an agreed specification. Always read the schedule of works.
Is the kitchen included?
Only if the quotation says so and defines the package. Kitchen supply is one of the largest swing factors between quotes.
Are planning and structural engineer fees included?
Professional fees are usually separate. We flag likely third-party costs early so they do not appear as a surprise mid-project.
Do your quotes include VAT?
We spell out VAT on every quotation so you can compare like with like. Most work to an occupied house attracts the standard rate.
Can I stage payment?
Yes. Payments typically follow agreed milestones linked to progress on site — not a huge upfront lump with nothing to show for it.
Why do quotes vary so much between builders?
Specification, exclusions, contingency, insurance, waste, making good and programme optimism all change the number. Compare the same scope, not just the bottom line.
Related
Useful next reads
Ready to discuss your project? Call us or request a quote today.
Request an extension quotation