If you want cheaper, quicker extra space that maximises daylight, a conservatory usually fits. If you need a fully integrated, energy-efficient additional room, an extension is normally the better choice. Conservatories cost less and go up fast, but an extension gives you a room that works as hard in January as it does in July. Planning and building-regulation tests can shift that answer for individual homes, so check the detail before you commit.
TL;DR:
- Conservatories are cheaper and quicker to build but generally provide limited thermal insulation and do not qualify as fully integrated living spaces.
- Extensions involve higher costs and longer construction times but offer better insulation, year-round comfort, and greater design flexibility for primary living areas.
- Planning permission varies according to property location, size, and previous modifications, with conservatories often permitted but requiring careful site-specific checks.
- Building regulations may exempt conservatories from certain standards if they meet specific criteria, but extensive upgrades can trigger full compliance requirements.
- Both options impact sustainability differently: conservatories use fewer resources initially but may have higher running costs, while extensions consume more material upfront but perform more efficiently over time.
Table of Contents
- Quick comparison: cost, disruption and planning risk at a glance
- Cost and build time: what drives the price in the UK
- Planning permission and permitted development: the checks that matter
- Building regulations and conservatory exemptions explained
- How much comfort can you expect through the year?
- Listed buildings and conservation areas: when to ask first
- How to decide: a checklist for comparing quotes fairly
- What to expect from feasibility through to handover
- Extension Kings: a managed route for homeowners choosing an extension
- Environmental impact and sustainability considerations
- What years of planning advice teach you about this decision
- How Extension Kings can help with your extension
- Sources
- FAQ
Quick comparison: cost, disruption and planning risk at a glance
Before you weigh design options, it helps to see the two side by side. Costs, timescales and planning risk all move in different directions depending on what you build and where.
| Factor | Conservatory | Extension |
|---|---|---|
| Typical cost | Lower per square metre | Higher per square metre |
| Build time and disruption | Shorter, less disruptive | Longer, more disruptive |
| Planning permission likelihood | Often permitted development | Depends on size, position and prior additions |
| Insulation and year-round use | Limited without upgrades | Built for year-round comfort |
| Design flexibility | Best as a bright ancillary room | Suits kitchens, bedrooms and open-plan living |
Three trade-offs sit underneath that table. First, cost versus permanence: a conservatory saves money now but rarely adds a room you can use as a bedroom or a kitchen. Second, light versus thermal comfort: glazing brings daylight but loses heat fast unless it is specified well. Third, speed versus integration: a conservatory can be built and used within weeks, while an extension takes longer but becomes part of the house rather than an add-on.
These figures are indicative. Site conditions, existing extensions and local authority interpretation all change the answer for a specific property, so treat this table as a starting point rather than a quote.
Cost and build time: what drives the price in the UK
Conservatories are generally the cheaper option per square metre because they use lightweight construction, minimal foundations and standard glazed panels. Extensions cost more because they involve full foundations, brickwork, roofing, structural steelwork where needed, and a complete set of building services. According to RICS consumer guidance, extension costs vary widely by property and scope, and early professional advice helps homeowners compare quotes on a like-for-like basis rather than by headline price alone.
When you request quotes, ask each contractor to itemise:
- Foundations and groundworks, including any need for structural calculations.
- Drainage, electrics and plumbing connections into the existing house.
- Thermal works: insulation, glazing specification and air tightness detailing.
- Joinery, internal finishes and decoration.
- VAT treatment and a contingency allowance for unforeseen work.
Build time follows the same pattern. A conservatory can often be erected in a matter of weeks once the base is in, while an extension typically runs from several weeks to a few months depending on scale. Contractor availability, groundworks conditions and weather all affect the schedule, so a fixed completion date is rarely realistic until the foundations are open and inspected.
Planning permission and permitted development: the checks that matter
Many single-storey rear extensions and most conservatories fall under permitted development, but the rules have specific limits that catch people out. Gov sets out the Class A limits, including the depth thresholds that separate a straightforward permitted development project from one that needs prior approval or full planning permission, and the requirement that extensions plus outbuildings do not cover more than half the curtilage of the original house.
Work through these checks before you commission any design:
- Confirm whether your house has used up permitted development rights through earlier extensions, since prior additions count towards the limits.
- Check the depth and height limits that apply to your proposed rear or side addition, including the neighbour consultation scheme for larger single-storey rear extensions.
- Establish whether your property sits on designated land, which removes some permitted development rights for conservatories and side extensions.
- Use the Planning Portal's tools to confirm your specific scenario, then contact your local planning authority for a formal view.
- Where the site is complex or heritage-sensitive, book pre-application advice before finalising a design.
Conservatories are often permitted development, but positioning matters: a conservatory built forward of the principal elevation, or on the side of a house on designated land, typically loses that right. Never assume a supplier's generic guidance covers your specific plot.
Building regulations and conservatory exemptions explained
Many conservatories are exempt from building regulations, but only under specific conditions. Planning Portal's guidance on conservatories and building regulations lists the usual criteria: the structure sits at ground level, covers no more than 30 square metres, is thermally separated from the house by external-quality doors, has independent heating controls, and has no new drainage connected.
- Removing the separating door to create open-plan space usually brings the whole structure into building-regs scope.
- Extending the house heating system into the conservatory, or adding plumbing, typically removes the exemption too.
- If a conservatory later becomes part of the dwelling, Approved Document L sets out the energy performance standards it then has to meet.
How much comfort can you expect through the year?
Large areas of glass lose heat quickly in winter and overheat in summer unless the specification is right. An insulated roof, thermally broken frames and correctly specified glazing U-values close most of that gap.
For an existing conservatory, practical upgrades include insulated roof panels in place of polycarbonate, higher-spec double or triple glazing, a dedicated heating zone with its own thermostat, and external blinds to cut summer solar gain. Extensions generally outperform conservatories on both thermal comfort and sound insulation because they are built with solid, insulated walls and roofs from the outset rather than retrofitted for comfort.
Listed buildings and conservation areas: when to ask first
Older and protected properties carry extra constraints that a standard permitted development check will not reveal. Historic England's advice on extending historic buildings notes that listed building consent may be needed even for a modest addition, and that any new work is judged on how it affects the building's significance and setting rather than on size alone.
Conservation area status, and what planners call Article 2(3) land, can also remove permitted development rights altogether and restrict materials, roof form and glazing style. If your property is listed or sits in a conservation area, speak to your local conservation officer or book pre-application heritage advice before you spend money on detailed design.
How to decide: a checklist for comparing quotes fairly
Start by defining what the space is actually for: a sunroom for reading is a different brief from a kitchen-diner you will use daily. Set a realistic budget that includes contingency, then check the planning and building-control risk for your specific plot before you fall in love with a design. Only then choose between an ancillary conservatory and a fully integrated extension.
On every written quote, ask for:
- Confirmation of who is responsible for planning and building-regulations applications.
- A clear insulation and glazing specification, not just "double glazed" as a catch-all.
- A realistic timescale with milestones, not just a start and end date.
- A staged payment schedule tied to progress, not a large upfront deposit.
- Written warranties on structural work and glazing.
Watch for vague allowances, undefined scope or promises made only verbally. Pro Tip: For complex or heritage sites, appoint an RICS chartered surveyor to assess the property before you commit to a design, as recommended in RICS consumer guidance.
What to expect from feasibility through to handover
- Feasibility and survey: 1 to 4 weeks to assess the site and confirm scope.
- Design and approvals: 2 to 12 or more weeks where planning permission is needed.
- Groundworks and foundations: 1 to 3 weeks depending on ground conditions.
- Structural build: 3 to 12 weeks for the main construction phase.
- Fit-out and snagging: 1 to 4 weeks to finish and resolve any defects.
Delays usually come from unclear scope, late surveys or long-lead items ordered too late, so agree the specification early and order glazing or steelwork as soon as the design is fixed. Tie payments to completed milestones rather than calendar dates.
Extension Kings: a managed route for homeowners choosing an extension
A managed design and build approach for various types of extensions across the West Midlands can coordinate design, planning and building regulations under one team. That managed approach sits directly against the checklist above: one point of contact for design, permissions and build reduces the coordination burden that RICS guidance flags as a common source of disruption on extension projects. Homeowners in Birmingham, Solihull and the surrounding area can view completed projects, including a kitchen-diner rear extension in Birmingham and a double-storey extension in Wolverhampton.
Environmental impact and sustainability considerations
Both options carry an environmental cost, but the balance shifts depending on materials and how the space performs afterwards. A conservatory typically uses fewer materials to build, which lowers its embodied carbon at construction. Its running costs can be higher relative to the space it provides if it needs supplementary heating for much of the year, because single-glazed or poorly specified units lose heat quickly.
An extension uses more material and labour upfront, including concrete foundations and brickwork, so its embodied carbon at the point of construction is higher. Once built to current insulation and glazing standards, though, it tends to perform better over its lifetime, needing less energy to heat and cool per square metre than an unimproved conservatory.
Material choice affects both routes. Reclaimed brick, responsibly sourced timber joinery and high-performance glazing units all reduce the lifetime footprint of either project. Rainwater harvesting, air source heat pump compatibility and solar-ready roof design are worth raising with your designer at the concept stage, since retrofitting them later usually costs more than specifying them from the outset. Whichever route you choose, ask your designer how the fabric specification compares against current Building Regulations thermal standards, since a structure that just scrapes past minimum requirements will cost more to run for decades to come.
What years of planning advice teach you about this decision
The cheapest option rarely stays cheapest once you count running costs and resale fit. Get planning and heritage risk checked before you fall for a design.
— Raja
How Extension Kings can help with your extension
If you have decided an extension is the right route, A managed process from initial design through planning, building regulations and construction by a single team can reduce the coordination burden for homeowners. This approach is used for kitchen extensions, wrap-around projects, side-return layouts and over-garage additions in the West Midlands.
- Browse recent extension projects across the West Midlands for real examples of finished work.
- Check region-specific cost guidance before you set a budget.
- Explore kitchen extension options if an integrated cooking and dining space is the goal.
Visit Extension Kings to request a quote and discuss your project with the team.
Sources
- Historic England advice on extensions and historic buildings
- Gov
- Planning Portal — Building regulations for conservatories
- Approved Document L, Volume 1 — Limiting heat gains and losses
FAQ
Is it better to have an extension or a conservatory?
It depends on how you plan to use the space. A conservatory suits a bright, lower-cost ancillary room, while an extension suits anyone who needs a fully insulated, year-round space such as a kitchen or bedroom.
Is £30,000 enough for an extension?
It can cover a modest single-storey addition in some cases, but RICS guidance notes that extension costs vary widely by property and specification. Get itemised quotes covering foundations, services and finishes before setting a fixed budget.
How much does a 3 metre by 3 metre conservatory cost?
There is no single UK-wide figure for a conservatory of this size, since price depends on frame material, glazing specification and roof type. Ask suppliers for an itemised quote that separates the base structure from any thermal upgrades.
What is the 50% rule for a house extension?
Under permitted development, extensions and outbuildings together cannot cover more than half the area of land around the original house, as set out in GOV.UK's permitted development technical guidance. Exceeding this threshold generally means you need full planning permission rather than relying on permitted development rights.