Aside from cost, understanding how long a home building project will take is often at the forefront of a client’s mind.

Trying to predict how long a home extension will take, or any other building project for that matter, is very difficult. This is because there are so many processes and variables involved, many of which are highly unpredictable.

As a rough guide, a single-storey extension project, inclusive of some refurbishment work to the existing building, will take about a year to complete if planning permission and party wall agreements are required. That’s the time from appointing an architect through to completion of the building work ready for occupation. The build phase will take about 3-4 months.

Below is a step-by-step guide for a typical extension project (design and construction) with approximately how long each stage will take for a typical residential project. It follows the industry standard RIBA Plan of Work, Stages 0-7.

The time it takes to complete each stage will obviously vary depending on individual circumstances, and so the estimates below are to be used as a general guide only.

Time-saving tips are included at the end of each section.

RIBA Stages 6 and 7 outline tasks following the completion of the building work for occupation and have been included here as they can have an impact on overall timescales.

Even if you don’t follow this guide stage by stage, it’s worth understanding the main tasks at hand as you will need to complete them in some form, either formally or informally, at some stage.

For advice on how to keep your project on track, check out our article - 10 tips to prevent your renovation project overrunning.

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RIBA Stage 0 – Strategic Definition

Time duration: 2-4 weeks

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Before committing to your project, consider whether building an extension is the right solution to your problem. Take time to explore all potential options. Organise your finances and consider timescales.

Ask yourself:

• Can I rework the existing space to provide what I need?
• Will my budget be sufficient to complete the work?
• Can I complete the project within the time I have available?

Architects don’t formally have a role during RIBA Stage 0. The purpose of this stage is to allow you, the client, to think freely about your objectives before you start to engage with professionals.

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At Designs in Detail, we are compiling a wide resource of information on residential architecture to help homeowners plan their building projects.

RIBA Stage 1 – Preparation and Briefing

Time duration:2-4 weeks

Stage 1 is the official start of the project. Once you are satisfied that building an extension is the right solution, the project can get underway.

Checklist:

• Prepare an outline brief setting out your main requirements and design ambitions. It will allow you to streamline correspondence when inviting prospective architects to quote for services.  
• Invite architects to quote for services. Finding the right architect can take time. Allow a period to carry out research on potential practices and arrange meetings in person to discuss your requirements and general goals.
• Agree a project brief. When architects are quoting for services, they will assess your requirements against budget and programme constraints, and advise if your goals are feasible.
• Agree the building route. The building route, or procurement strategy, sets out the roles and responsibilities for the design team. It can also influence the level of detail an architect and other consultants will supply at the project’s various stages. Again, architects will advise during pre-appointment discussions.
• Appoint the design team. If you want to build a good-quality residential project, this usually means appointing an architect in the first instance. Other consultant appointments can happen later, which may include a structural engineer, quantity surveyor and party wall surveyor. Read more on how to appoint an architect in our article - Do I need an architect?
• Confirm health and safety duties. Under the Construction (Design Management) Regulations 2015 (CDM), a domestic client’s duties can either be passed on to a designer (architect or other) or the builder. An architect will advise how best to proceed when formalising their appointment.
• Finally, commission a measured survey and carry out site investigations to prepare the way for the start of the design phase. The architect can organise this on your behalf.

Time saving tip: If you have agent plans, the architect can begin sketching layout options while the site information is being prepared.  

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RIBA Stage 2 – Concept Design

Time duration:2-8 weeks

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Following completion of the brief and site information, your architect will begin design work and will organise other consultant appointments as needed.

Checklist:

• Preliminary options for your comments. These usually take the form of rough sketch layouts, visualisations and working 3D models to explore the different design possibilities.
• Review planning policies and advise on planning route. This will involve doing a search of local approvals and analysing key planning policies. You may be able to build without planning permission if your proposals meet permitted development guidelines.
• Pre-application planning advice. You may wish to consider applying to your council for pre-application advice if you’re proposing a large extension (say over two storeys) or building in a sensitive location. Councils will normally respond within 4-6 weeks.
• Appoint other specialist consultants required to develop the design. This usually requires the input of a structural engineer. For budgets exceeding £150K, it’s advisable to appoint a quantity surveyor to provide expert cost advice.
• Agree the final project brief and select a preferred design option for further design development.

 Time saving tip: If you’re on a tight programme, only apply for pre-application advice if it’s necessary. For many types of home extension, a good architect will be able to use their experience and advise on what is likely to be acceptable in planning terms.

RIBA Stage 3: Spatial Coordination 

Time duration: 2-12 weeks

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For residential projects, this is typically the stage where you develop a preferred design for permitted development or planning approval.

Checklist:

• Draw up the preferred design in CAD. This will include the preparation of plans, sections and elevations. Depending on the architect’s fee, it may also include a more refined 3D model and visualisations.
• Carry out an outline structural design and submit for cost analysis. Before you commit to submitting a design for planning approval, it’s often advisable to appoint a structural engineer to prepare an outline structural design as this will help define budget costs. Once complete, you can then appoint a quantity surveyor to produce a cost estimate based on approximate measured quantities, and/or tender the information out to builders for initial pricing. You can firm up build costs with the builder once the technical design is complete.
• Consider construction methods and sustainable strategies. If there are any building systems you’re interested in using, now is the time to investigate.
• Check the design against the project brief to ensure your objectives are being met. The design process can throw up lots of different potential options and it can be easy to lose sight of your objectives. Referring to the project brief will help keep you on the right track.
• Prepare planning documentation and submit plans for planning consent. This will include refining the final design drawings as necessary following cost analysis and preparing the other documentation for the planning application.
• Planning period. Planning applications for residential extensions are normally decided within 8-10 weeks. Decisions can take longer if, say, alterations are required. Read more about how long planning takes in our article - How long does planning take?

Time saving tip: Check if your proposals require planning permission. Many extensions can be built under permitted development, meaning you may not need to factor in a planning period. If planning is required, appoint an architect to submit on your behalf to make sure the process runs as smoothly as possible.

RIBA Stage 4 – Technical Design

Time duration: 8-24 weeks

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For home renovation projects, Stage 4 can be separated into three distinct parts: i) preparation of technical drawings and specifications, ii) party wall agreement, iii) tender and builder appointment.

Party wall agreements are only required if you are building close to a neighbouring building or shared boundary. If this does not apply to your project, the stage can be completed a lot sooner.

Stage4 (i)- Technical Drawings and Specifications, Time duration 4-6 weeks.

Checklist:

• Prepare detailed drawings and specifications. These will set out the demolition works, drainage runs, construction build-ups, internal and external materials, finishes and fittings, and electrical provisions. The level of detail will vary depending on your chosen building route.
• Co-ordinate design with the structural engineer and any other consultants. For residential projects, this usually involves making sure the structural engineer’s detailed design works with the other aspects of the build. If you have appointed any other specialist consultants – audio visual, lighting etc – their work will have to be coordinated to make sure the whole design works seamlessly together.
• Co-ordinate design with specialist subcontractors. Glazing companies, for example, may need to supply shop drawings to confirm how their component parts work with the build.
• Discharge planning conditions. Some planning permissions are subject to pre-commencement conditions which need to be discharged before building work can start. Conditions are normally discharged within 8 weeks.
• Make an application for building regulations approval, either full plans application or building notice depending on scope of work. Note, this only signs off the design. Full building control approval is granted when the site work is complete.
• Prepare pre-construction health and safety information. This may include designer’s risk assessments and the preparation of a health and safety file depending on the number of contractors on site.  

Time saving tip: If your extension can be built under permitted development, or you feel planning permission is likely, you may consider starting Stage 4 at risk during the planning period to compress the overall programme.

Stage 4 (ii) Party Wall, Time duration 1-12 weeks.

Checklist:

• Prepare information for party wall award and appoint surveyor (if applicable).This will involve drawings of the boundary conditions and serving the correct notices to neighbours. Select information prepared in part (i) will normally suffice. With the guidance of your design team, you may be able to submit the notices yourself. For more complex agreements, you will need to appoint a party wall surveyor to act on your behalf.
• Party Wall period. This period begins when you serve notice to your neighbours. The main building work cannot start on site until you have reached an agreement with your neighbours. If your neighbours are happy for the works to go ahead, the party wall period can be completed very quickly. If they dissent, it can take several weeks to resolve.

Time saving tip: Discuss your proposals with your neighbours before you submit your plans for planning permission (they will be notified). Explain the design in person and advise that you will rectify any damage to their property that may be caused by your works. If you can get them on side, you’ll be able to save a lot of time and expense.

Stage 4 (iii) Tender and Builder Appointment, Time duration 4-6 weeks.

Checklist:

• Prepare a schedule of works. This is a pricing document that is sent to the prospective builders for completion. If you do not prepare a schedule of works, each builder will create their own pricing document making it difficult to analyse prices. For example, formats can vary and work items can be grouped differently.
• Tender period. Send the schedule of works and technical information (including any information from specialist suppliers) to the builders for pricing. Four weeks is normally sufficient time for tenders to be returned.
• Analyse tenders and select your builder. Compare prices, arrange to view completed projects, and select your preferred builder. If build costs are too high, adjust the design to bring your project back on budget.  
• Agree build cost estimate, prepare the building contract and appoint your builder. Finally, before works begin on site, agree the final cost estimate of the works and sign a building contract. The building contract will not only set out the cost of the works, and start and completion dates, but also the roles and responsibilities of the parties involved. The type of contract will be dependent on your chosen build route.
• Preparation of construction phase plan. The builder is to prepare a construction phase plan as required under CDM regulations.

Time saving tip: Reach out to potential builders once you have received planning permission – the point at which the general parameters of the design are set. Good builders are in high demand and the sooner you can get one lined up, the better.

RIBA Stage 5 – Manufacturing and Construction

Time duration: 12-16+ weeks

RIBA Stage 5 covers the construction phase. The builder takes possession of the site and carries out the works in accordance with the construction information. This includes any manufacturing off-site.

The time it takes to build an extension varies according to the design, site and extent of renovation work to the existing building. As a rough guide, a typical single extension – inclusive of some ground floor renovation work – will takeabout 12-16 weeks to build. A two-storey extension, or complex single storey extension, forming part of a whole house refurbishment can take a year.

Checklist:

• Site visits to check work is being carried out in accordance with the plans. This is only applicable if you have retained the services of an architect and are using a traditional procurement route. If the works are being carried out on a design and build procurement and the builder is project managing the work, this may not be practical.
• Liaise with the council or private building control services as necessary. Key stages of the build need to be inspected and signed off by the building inspector to gain building control approval. The builder will typically organise site visits. The architect will liaise on design matters if you have retained their services.
• Value site works, document defects and arrange for their rectification. Organise inspections for commission and testing, sign off work for practical or partial completion, and issue certificates as appropriate. Again, the architect will carry out this service as part of a traditional building route. For design and build procurement, the builder is largely free to deliver the build without professional oversight.

Time saving tip: When time is of the essence, Stages 4 and 5 can progress simultaneously. This means work starts while the technical design is still being completed. You will need to accept higher price uncertainty (fewer details will be resolved) but you can reduce the overall programme. The trade-off is therefore between speed and cost certainty. The more detail you provide before site work starts, the greater the price certainty – but the longer the overall programme.  

RIBA Stage 6 – Handover

Time duration: 6-12 months

RIBA Stages 1-5 set out the order of events from start to completion of the building work for occupation. However, to answer the question ‘how long does a home extension take?’, we must also consider tasks that may need to be carried following occupation – often referred to as Practical Completion.  

Stage 6 signifies the end of construction and the handover of the site from builder to client. Most aspects of the building contract are concluded, but a few tasks remain.

Checklist:

• Handover of the health and safety file. To comply with CDM regulations, the builder should hand you the file when the site is ready for occupation. It should include information for future repair and building work including operational instructions for the building’s use, provisions for cleaning and maintenance, and information on the building’s construction for any future construction works.
• Defects Liability or Rectification Period. The builder should fix any defects found within this time and a sum may be withheld repayable until the end of the period is reached – normally between 6 and 12 months depending on the building contract. For a traditional building route, a Final Certificate is issued to start the defects liability period and a Completion Certificate to signify it is sent.
• Testing and Commissioning. Organise with the builder and suppliers for the testing and commissioning of all building systems, fixtures and fittings to enable the building to function properly. Activate warranties and guarantees.

RIBA Stage 7: Use

Time duration: Ongoing

The final RIBA stage focuses on the day-to-day use of the building and was brought in to help clients and design teams evaluate whether the design is working as intended. It also deals with the life cycle of buildings, including future alterations or additional works.

Stage 7 services normally fall outside of the original scope of work and separate appointments may be needed.

Checklist:

• Building management. This includes monitoring the building’s performance to check whether the sustainability strategy is working successfully and advising on any adjustments that may be required.
• Appraisal of built works. Following occupation, appraise with the design team whether the completed works perform as intended. Instruct any additional works required so the building meets your requirements.
• Future alterations or additional work. Your extension may be part of a wider development plan. For example, there may be other parts of the house you would like to refurbish or outbuildings you would like to convert in the future. Discuss potential options with the design team to work through the best course of action.

FAQs

What is the RIBA Plan of Work?

The RIBA Plan of Work is an industry standard document that organises building projects into key work stages and sets out the main outcomes and tasks for each stage. It covers the briefing, design, construction and operational use of building projects.

Do I need to follow the RIBA Plan of Work?

No, but it will provide greater clarity and structure to your project. It has taken industry experts many years to develop and will give you the best chance of delivering a successful project.

How long does it take to build an extension?

A simple, single-storey extension will take about three to four months to build. Larger, more complex extensions, such as a two-storey addition, are likely to take more than six months to build.  

How quickly can I build a loft conversion?

A simple rear dormer loft conversion can take about six weeks to build.

How long does it take to build a basement extension?

A standard residential basement extension will take around 12-24 weeks to build subject to size, ground conditions and fit-out specification. Converting an existing space, such as a small cellar, can take as little as 4 weeks. Basement works can be highly complex and should be carried out by specialist contractors.

How do I stop my home extension project overrunning?

Appoint a professional design team –architect, structural engineer, plus others as required – to design and manage the project on your behalf and use a standard form of building contract to cover the site works. Find more guidance check out our article - 10 tips to prevent your renovation project overrunning.

Do I need to move out while I build an extension?

If the building works are confined to a single part of your property, it’s possible to live on site while they are taking place. However, it may not be possible if your extension is part of a whole house refurbishment. In such circumstances you should consider arranging temporary accommodation.

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Credit - Main image - Photo by 2H Media on Unsplash

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About the author: Aron Coates is an architect with over two decades of experience, encompassing projects ranging from the restoration of historic buildings to the design of contemporary homes.

The information provided in this article is intended for general guidance and educational purposes only. At Designs in Detail, we advise you appoint a skilled residential architect to provide specific expert advice for your project.

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