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  • Writer's pictureSam Nolan RIBA Architect

How Much Will It Cost?

Updated: Aug 4, 2021

One of the first questions we are asked by clients at initial meetings when looking at a new project is how much is it likely to cost. So how much then?!

With so many variables involved working that out is not a straight forward answer that can be worked out on the spot at an initial meeting. The only way to derive any kind of certainty of cost is to go to the market and obtain quotations or a range of tenders for the works but there are estimating, budgeting and cost control measures which can be undertaken at various pre-tender and construction stages of the project.

At an initial meeting without a sketch design or worked through scheme it is only really possible to quote an approximate cost per square meter allowance based on experience of recent similar projects. That has to be heavily caveated as it usually doesn’t allow for expensive features, fixtures or finishes such as large areas of glazing, sliding/ folding doors, wood burning stoves, expensive kitchens of floor or wall finishes. A kitchen could cost either £10,000 or £50,000 depending on specification, similarly floor finishes could range from a basic specification of £30m2 to higher end £100m2.

As a rough guide the below rates could be used as a starting point for working out your initial project construction budget:

New Build House Standard Construction and Specification: £1,500 to £2,000 per m2

New Build House High Specification: £2,500 per m2

Barn Conversion (Standard Specification to high): £1,500 to £2,500 per m2

Single Storey House Extension: £1,650 to £2,000 per m2 of new floor area

(if your new extension will be 5m x 4m in length, that will be 20m2 in area)

- As a basic rule of thumb a two-storey extension of the same size at each floor could cost 50% to 75% more than a single storey extension. The average cost of a two storey extension is £1,750 per m2.

- A typical cost of a simple two storey side extension for a 5m x 4m (40m2) could be in the region of

£70,000 + VAT.

Loft Conversion: £1,000 to £1,500 per m2 Dependant on extent of works and structural alterations.

Internal refurbishment works: £600 to £1,200 per m2 depending on extent of works and specification of finishes.

Once the works have been defined more clearly though a sketch design scheme and planning drawings a budget estimate can be obtained from a cost consultant to give a more defined picture of the likely cost of the works. That can be further refined by a pre-tender estimate once the detailed design for the project has been completed. If the works are over budget at pre-planning or pre-tender stage a value engineering or cost saving exercise can be undertaken to try to bring the works back within budget. That may include reviewing the size or scope of the project, revising or omitting selected items of work, or reviewing the specification for the project. Please ask us about engaging the services of a cost consultant and we can give an indication of costs and arrange that on your behalf.

Additional features may cost:

- Bifold Doors: allow approximately £1,000 per linear meter or leaf

- Wood or solid fuel burning stove including surround: £2,500 - £4,000

- New kitchen costs could range from £10,000 to £50,000 (or more!)

- A new Bathroom could add in the region of £5,000 to £7,000

- New structural openings: £3,000 to £10,000 (dependant on what supporting & span)

Abnormal items & conditions which may affect costs:

- Drainage location and diversions: If the works are close by or over a public sewer a build over agreement may be required with the statutory undertaker and additional works to renew existing sewers may be required.

- Ground Conditions: Poor ground conditions can affect foundation design. For larger projects ground investigation is essential and usually requested by the project engineer.

- Trees: (If certain types of trees are located close to the proposed building works foundations may have to be deeper to allow for ground movement or shrinkage).

- Asbestos: Is the property of an age where asbestos containing materials may have been used in its construction or subsequent alterations? If so an asbestos demolition and refurbishment survey should be allowed for and undertaken prior to the commencement of work. If asbestos is found there would be the cost of strip out works. Asbestos can be found in any residential or industrial building built or refurbished prior to the year 2000.

- Party Wall Issues: Proximity to neighbours and boundaries if the proposed works are located close to a neighbouring property or astride a boundary the may require a party wall award issued by a party wall surveyor.

- External works: Are there likely to be external works and landscaping required, would existing external areas and gardens need to be reinstated or made good following completion, if so allowances should be included.

- Site preliminaries and access: Is the site access and parking good and can plant and machinery access the site easily? Is there enough room on the site for storage of materials and contractor’s compound and welfare? All of those can have an effect on costs if other provisions have to be made or smaller machinery hired in or work carried out by hand.

- Procurement method: We would always recommend a building contract is put in place for any building works to minimise financial, quality, time and health and safety risks but some clients with previous building experience are happy to undertake project management themselves. This can be a significant saving on professional fees particularly for smaller projects where the fee for tendering and contract administration services can appear to be disproportionate to the overall construction cost. If you are considering project managing building work then please ensure you are aware of the possible risks involved. Please ask us for further information on that if you have any queries on this issue.

Similarly, if some of the building work would be undertaken by you rather than your builder or outside of a contract by others that would reduce costs.

Other items which should be considered in your overall project budget:

VAT: The dreaded 20% VAT. New build residential projects and charitable ones (as long as they do not generate any income) do not at the time of writing attract VAT. Extension, refurbishment and repair works do. For a project to be considered new build residential the existing building must be demolished to the ground floor slab prior to the commencement of the new build. There are reduced rates of up to 5% that apply to certain types of work such as bringing empty properties back into use. If you are converting an existing non-residential building such as an office or barn to residential use then you are creating a new dwelling and the 5% rate may apply. To confirm the VAT payable on your project and if there is any question regarding the amount of VAT you think your project would incur advice should be sought from a VAT specialist accountant.

If a project is standard rated for VAT purposes, given the 20% rate a project with a construction budget of £100,000 has an actual construction budget of £83,333 once VAT has been allowed for.

Professional fees: Professional and application fees depend on the size and type of project but can amount up to more than 10% of the construction budget. Professional services and fees in addition to architects’ fees which may be required include: topographical surveyor fees, structural engineers fees, ecology reports, CDM/ Health and safety principle designer fees, party wall surveyor fees, cost consultant fees and landscape design. Application fees can include local authority planning application fees, building control plans and site inspection fees.

Architects fees: Architects fees are dependent on the level of service and the scope and complexity of the project. Fees can be worked out in several different ways which could include percentage fees, where the fee is worked out as a percentage of the construction cost or an estimate of the time and cost involved in the work at each stage. Lump sum fees are rarely used and then only when the extent of work is absolutely clear and the exact time scale of the service is known from the start. The standard hourly rate for most architects with more than 5 year’s experience in a professional practice is around £75 per hour. VAT is usually payable on architectural fees. Architects are qualified building professionals who have undertaken seven years of training and work to the highest standards in design, construction knowledge, health and safety and quality assurance no other building professionals are trained in design and construction to such a level of expertise. They are regulated by a statutory body and required to retain professional indemnity insurance in respect of work carried out and services performed.

Insurance Costs: Prior to the commencement of work to an existing property the client must contact their home insurer and advise them of the works. Contract works insurance should be put in place in addition to checking that your contractor carries the correct contractors and public liability insurance policies. There may be an increase on the home owners premium depending on the extent and nature of the works and existing property.

Contingency: It is recommended to allow for a contingency of at least 5% to cover any unforeseen issues. This may rise to 10% when dealing with an existing or listed building depending on the complexity of works.

We hope the above information is a useful starting point and introduction to construction costs at an early stage of your project but if you have any questions on any of the above information then please do not hesitate to contact us and we would be happy to assist.

How Long Will It take?

The next questions we are asked is how long will it take. So how long then?!

Again, with so many variables involved working that out is not a straight forward answer that can given straight away. A typical project programme for a simple extension is shown below but every project is different and there is no guaranteed time line for any residential extension project. Other factors which may affect timescales are:

Finding the right builder for the right price at the right time. Builders are still very busy, with some booked up for more than a year in advance so finding a slot with your selected builder can have an effect on timescales.

Design decisions. During the briefing and design stages changes to requirements or the design may take additional time to reconcile and produce. A clear brief from the outset and keeping any changes to a minimum can save time.

Planning issues: if the site is a contentious site or if planners require additional information that can add time to the planning process.

Building Control: if the build is a complex build and would involve complex structural engineering requirements that may add cost and time to the project. Similarly, at construction stages, a simple build will take less time and carry less risk of additional cost.

Construction: The key to saving time during construction is having a good set of thoroughly worked out information available for the contractor. Any unresolved items or late changes are likely to add cost and time to the process. It is worth giving consideration to the time of year you start work on site as other unforeseen issues such as bad weather may affect time scales at construction stage, it goes without saying that November/ December may not be the best time to commence works on site.

House Extension Possible Time Scales:

In the interests of providing realistic information and managing expectations some indicative timescales for a simple house extension are shown below. It may be possible in some circumstances to expedite some of the stages to reduce timescales if required but similarly some circumstances can lead to stages taking longer.

RIBA Stage Possible Time

Initial Client Contact, Visit, Proposal of Service/ Fee Quote

2 Weeks

Information gathering, Site & Building Survey & Services Searches

2 to 4 Weeks

Concept/ Sketch Design Stage: Allow time to produce the design, client discussions and any subsequent amendments.

Up to 4 Weeks

Planning Stage: Planning Application: Allow 4 weeks to produce planning CAD drawings and allow for client discussions & changes, 2 weeks for local authority to register the application and then an 8 week determination period.

Up to 14 Weeks

4 Technical Design

Technical Design & Building Control Application: Allow 4 weeks to prepare the building control information and then an 6 week building control plans application determination period.

Up to 10 Weeks

Tendering/ Pricing Stage: Allow 4 weeks to prepare tender information and then 4 weeks for contractors to price the works. Allow time for tender analysis, reporting and appointing and any further negotiations with contractors.

Up to 8 Weeks

Construction Stage: Works on site for a simple extension could take anywhere between 8 to 16 weeks depending on the nature of the work and the size of the building contractor selected. A smaller one man builder may be less expensive but may take longer than a larger contractor with more available resources.

From 8 to 16 Weeks

Completion & Handover: 2 Weeks

Possible total time to completion: Up to 60 Weeks

The above timescales are indicative and may be affected by other circumstances specific to your project.

As the timescales show it is worth starting to plan your project more than 6 to 8 months in advance of when you would like to start on site and realistically to allow for more than a year for the entire process from inception to completion.

All information on costs and timescales is correct at the time of writing January 2021. Please check for current and updated information at the time of your enquiry.


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