How Long Does a House Extension Take from Planning to Completion?
A simple extension can sometimes be completed within several months, but the full programme depends on the design, approvals, builder availability and the complexity of the construction work.
When homeowners begin considering an extension, one of the first questions is usually: how long will the entire project take?
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The honest answer is that it depends on the property and the type of extension.
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A modest single-storey rear extension may progress relatively smoothly. A large wraparound extension with extensive structural alterations, drainage diversions, bespoke glazing and a new kitchen will naturally take longer.
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The construction period is only one part of the programme.
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Before a builder can start work, you may need to develop the design, obtain planning permission or confirm permitted development rights, prepare Building Regulations drawings, arrange structural calculations, deal with Party Wall matters and obtain comparable builder quotations.
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For a straightforward extension, it is sensible to allow approximately six to nine months from the initial design stage to completion.
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For a larger or more complex project, the full process may take nine to twelve months or longer.
A Typical Extension Timeline
Every project is different, but the following provides a useful starting point:
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Project Stage / Typical Allowance
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Initial consultation, survey and design development / 3–6 weeks
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Planning application where required / 8 weeks after validation
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Building Regulations drawings and structural design / 4–8 weeks
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Builder quotations and appointment / 3–6 weeks
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Party Wall process where required / Allow at least 1–2 months
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Construction of a straightforward extensionApproximately 10–16 weeks
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Larger extension or extensive refurbishment / Approximately 4–6 months or longer
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Some stages can overlap.
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For example, Building Regulations drawings and structural calculations may be prepared while the planning application is being determined, provided the homeowner is comfortable progressing at risk.
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Similarly, builders can sometimes be approached before every technical detail has been finalised, although detailed quotations will normally be more reliable once the construction information is available.
Stage 1: Exploring Your Options
Before drawings are prepared, it is worth spending time deciding what you actually want the extension to achieve.
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This might include:
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A larger kitchen and dining area
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Better access to the garden
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A home office
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A utility room
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A downstairs WC or shower room
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A playroom
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An additional bedroom
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Future-proofed living space
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A combination of an extension and loft conversion
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At this stage, consider the available garden space, the type of property, your budget and whether the project should be modest or more ambitious.
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A clear brief can save time later.
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Changing the design repeatedly after a planning application has been prepared or after builders have priced the work can delay the project and increase costs.
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Stage 2: Survey and Initial Design
The next step is usually a measured survey of the existing property followed by preparation of the design drawings.
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For a straightforward extension, the initial design stage may take several weeks.
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The drawings may include:
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Existing floor plans
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Proposed floor plans
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Existing elevations
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Proposed elevations
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Roof plans
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Site or block plans
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Basic dimensions
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The relationship with neighbouring properties
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The location of doors, windows and rooflights
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The design may need to be adjusted after the homeowner has reviewed the first draft.
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It is better to resolve the layout properly at this stage than rush ahead with a design that does not make the best use of the property.
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Stage 3: Planning Permission or Permitted Development
Some extensions require a householder planning application.
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Others may fall within permitted development rights, subject to the size, height, location and design of the proposal and the circumstances of the property.
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Planning permission may be more likely to be required for:
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Larger rear extensions
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Wraparound extensions
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Double-storey extensions
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Extensions to flats or maisonettes
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Listed buildings
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Homes in conservation areas
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Properties where permitted development rights have been restricted or removed
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Designs that exceed the normal permitted development limits
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A typical householder planning application should usually be decided within eight weeks after it has been validated by the local planning authority.
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Validation is important.
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If information is missing or the drawings do not satisfy the local requirements, the council may not validate the application immediately. The eight-week determination period will not normally begin until the application has been accepted as valid.
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It is sensible to allow additional time in the overall programme rather than assume that approval will arrive exactly eight weeks after the drawings are submitted.
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What About Larger Rear Extensions?
Some larger single-storey rear extensions may use the prior-approval process rather than a standard householder planning application.
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This process applies only in specific circumstances and involves consultation with adjoining neighbours.
The correct planning route should be checked before work begins.
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What If the Planning Application Is Refused?
A refusal does not always mean the project must end.
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Depending on the reasons given by the council, it may be possible to revise the design and submit an amended application.
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However, this can add several weeks or months to the programme.
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Where a proposal is likely to be sensitive, it is worth taking planning advice early and adopting a realistic approach to the size and appearance of the extension.
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Stage 4: Building Regulations Drawings
Planning drawings and Building Regulations drawings serve different purposes.
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Planning drawings explain the size, layout and external appearance of the extension.
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Building Regulations drawings provide more detailed information about how the extension should be constructed.
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Depending on the project, the technical package may cover:
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Foundations
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Walls
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Floors
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Roof construction
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Insulation
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Ventilation
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Drainage
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Fire safety
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Doors and windows
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Structural openings
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Materials
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Thermal performance
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Electrical and plumbing considerations
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Preparing the Building Regulations package may take several weeks.
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A full-plans Building Regulations application is usually preferable for an extension because it allows the technical design to be reviewed before construction progresses too far.
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The formal decision period is normally five weeks, although this may be extended to two months by agreement.
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Stage 5: Structural Design
Many extensions require a structural engineer.
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Structural calculations and drawings may be needed where the project involves:
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Steel beams
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Large openings in the rear wall
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Removing load-bearing walls
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Supporting walls above new openings
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Altering the roof
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Building a double-storey extension
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Complex foundations
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Unusual ground conditions
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Removing chimney breasts
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Combining an extension with major internal alterations
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The structural design often develops alongside the Building Regulations drawings.
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The programme will depend on the complexity of the work and the engineer’s availability.
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Where the extension includes large structural openings, it is sensible to complete the calculations before asking builders for detailed quotations. Otherwise, the builder may need to make assumptions that affect the accuracy of the price.
Stage 6: Party Wall Matters
Some extensions require notices under the Party Wall etc. Act 1996.
This may be relevant where the work includes:
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Excavating close to a neighbouring building
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Building on or near the boundary
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Cutting into a shared wall to insert steel beams
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Raising or altering a party wall
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Carrying out certain structural works close to an adjoining property
Not every extension requires a Party Wall Award.
However, if the Act applies, the issue should be addressed before construction begins.
The minimum notice period will depend on the work involved. As a general guide, work to an existing party wall usually requires at least two months’ notice, while new boundary structures and certain excavation works generally require at least one month’s notice.
If the neighbour consents promptly, the process may be relatively straightforward.
If surveyors need to be appointed and a Party Wall Award prepared, additional time should be allowed.
Stage 7: Obtaining Builder Quotations
Homeowners often underestimate how long it takes to obtain and compare builder quotations properly.
A reputable builder may need time to review:
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Design drawings
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Building Regulations drawings
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Structural calculations
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Drainage information
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The written specification
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The proposed finishes
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Access arrangements
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Site conditions
It is usually sensible to obtain several quotations and compare them carefully.
This may take three to six weeks, sometimes longer if the builders are busy or the project is complex.
Avoid choosing a builder solely because they can start immediately.
A good builder may already have work booked several months ahead. Availability matters, but it should be considered alongside experience, communication, references and the quality of the quotation.
Stage 8: Ordering Materials and Specialist Products
Some products have longer lead times than homeowners expect.
These may include:
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Sliding or bifold doors
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Bespoke windows
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Roof lanterns
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Rooflights
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Structural steelwork
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Bricks to match the existing house
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Kitchens
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Worktops
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Specialist tiles
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Underfloor heating systems
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Bespoke joinery
The builder should identify critical items early and place orders at the appropriate stage.
A project can lose several weeks if a large glazed door, roof lantern or kitchen is ordered too late.
Stage 9: Construction
Once the builder starts on site, a typical extension may involve:
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Site setup and protection
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Demolition and strip-out work
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Excavation and foundations
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Drainage alterations
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Ground-floor construction
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External walls
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Structural steelwork
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Roof construction
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Doors, windows and rooflights
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First-fix plumbing and electrics
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Insulation and plastering
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Second-fix work
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Kitchen or bathroom installation
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Flooring and decoration
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External making good
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Snagging and final inspections
A straightforward single-storey extension may take approximately 10 to 16 weeks on site.
A larger wraparound extension, a double-storey extension or a project involving extensive internal refurbishment may take four to six months or longer.
Weather, access, ground conditions, structural discoveries and changes requested during the work can all affect the programme.
What Can Delay an Extension?
Even a well-planned project can experience delays.
Common causes include:
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Planning validation issues
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Design changes
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Planning refusal and resubmission
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Slow decisions on finishes
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Party Wall disputes
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Builder availability
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Structural changes
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Deep foundations
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Unexpected drains
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Poor ground conditions
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Asbestos
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Material shortages
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Delayed windows or glazing
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Kitchen lead times
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Bad weather
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Additional work requested during construction
Older properties can be particularly unpredictable.
A Victorian or Edwardian house may contain drainage runs, previous alterations or structural details that were not obvious before the work began.
A sensible programme should include a degree of flexibility.
Can Any of the Stages Overlap?
Yes.
A carefully managed programme can save time by allowing some tasks to progress together.
For example:
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Building Regulations drawings can be developed while planning permission is being considered
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Structural calculations can be prepared alongside the technical drawings
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Party Wall notices can be served before the proposed construction date
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Builder discussions can begin before every product has been chosen
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Windows, doors and kitchens can be ordered in time for the relevant construction stage
However, there is a balance.
Moving too quickly can lead to wasted work if the design changes or planning permission is refused.
The right approach will depend on the project and the homeowner’s appetite for risk.
Should You Book a Builder Before Planning Permission Is Granted?
You can begin speaking to builders before planning permission is granted.
This can be useful if you want to understand likely costs, availability and practical construction issues.
However, it is sensible to distinguish between an early estimate and a firm quotation.
A builder may not be able to price the project accurately until the design, Building Regulations drawings and structural calculations are sufficiently developed.
Before paying a substantial deposit or agreeing a fixed start date, make sure the necessary approvals and technical information are in place.
A Realistic Programme Is Better Than an Optimistic Promise
Most homeowners would like their extension completed as quickly as possible.
However, rushing the early stages can cause problems later.
A few extra weeks spent refining the design, preparing detailed drawings, obtaining structural advice and comparing builder quotations properly may save money and reduce delays during construction.
The best programme is not necessarily the shortest programme.
It is the one that allows the project to progress in a clear, organised and realistic way.
Start Planning Your Extension
A well-designed extension can transform the way your home works, but it requires careful preparation.
Depending on the project, you may need architectural design support, planning advice, Building Regulations drawings, structural calculations, Building Control input, Party Wall advice and an experienced extension builder.
Tell us a little about your property, the space you hope to create and the stage you have reached. We can help connect you with suitable professionals for the next steps.