A real project example Cost Benchmarking

The adjacent page is an example sheet from a cost benchmarking exercise for a House in Leytonstone, London.

The clients are seeking to add a full width rear extension with a living/ dining/ kitchen room that integrates into the existing house footprint adding both a ground floor wc and utility room. The existing living room and corridor spaces will be upgraded.

Using our benchmarking strategy the floor plan was split into three zones and we applied the cost estimate range per zone as follows:

Extension: ~20 sqm = £60,000 - £80,000

Existing refurbishment: ~16 sqm = £32,000 - £48,000

Light refurbishment: ~16 sqm = £8,000 - £16,000

Using estimated areas based on the estate agent plans, the net construction cost ranges from £100,000 - £144,000. 

Conclusion

We will explain additional costs in more detail in the journal below, however the client here wanted higher level finishes and a bespoke extension design. The net construction budget was set at £140,000.

Construction budget - £140,000

VAT (at 20%) - £28,000

Surveys, professional and statutory fees - £25,000

Total budget - £193,000

Budgeting for a project beyond net construction cost

Understandably, clients want to know the "all in" total cost of a project. The previous budgeting exercise is for net construction costs but above this you also have VAT, surveys, professional fees and statutory fees. Additionally, money may need to be set aside if there is requirement for new Furniture, Fixtures & Equipment (FFE) - these are soft furnishings that are typically not part of the Total Sum in the Construction Contract.

As a rule of thumb, clients should allow 20% VAT to be added to net construction cost and anticipate an additional sum of 15-20% for all surveys, professional and statutory fees.

We will run through each additional cost in detailed sections below.

Surveys, Professional Fees and Statutory Fees

During the process, you will need to engage and instruct other consultants and specialist surveys. See the list below and some benchmark figures for estimating purposes only (figures all exclude VAT). Please note that the list is not exhaustive and was drafted based on my experience with similar projects in London.

Required Services:

Full Measured Survey - First step is always to get a full measured CAD survey of the house. £1,500 - £2,000. 

Drainage Survey - If there are issues with the below-ground drains, it is best to identify these early with a survey. In addition, you may need information regarding manholes as any work affecting a public sewer may require a build-over agreement. £400 - £600.

Structural Engineer - You will need a structural engineer who will provide the necessary structural drawings and calculations for the tender pack, building control, party wall, and construction. £3,000 - £4,000.

Building Control Officer - Typically we recommend using a private firm known as an Approved Inspector. As soon as the detailed drawings are ready they will be issued to the BCO for an initial check, they also visit the site during construction to inspect and approve that the works are being completed according to the regulations. I would allow £1500-2500.

Read more on Party Wall and Building Control on our journal post:

Guide to Building Control, Party Wall Agreements and Principal Designer Role.

Party Wall Surveyor - You will need a party wall surveyor for any work on a shared wall or garden wall. The basic cost for a notice to be issued is £250 and these will need to be issued to all the attached freeholders and leaseholders. However, if an award is required, the costs can range between £1,000 - £5,000 on average.

Planning Applications

For information on full planning applications and permitted development costs, please refer to our detailed post:

Guide to Planning Permission and Permitted Development in London.

Optional Services:

Quantity Surveyor - If desired, for a more accurate understanding of costs early on then we recommend an Initial Cost Plan. £1,000 - £1,500. There is also the option to use a QS for the full service until the end of the project, this would cost £8,000 - £10,000 and may be best suited to higher valued projects.

Landscape Designer - A landscape consultant can re-image your outdoor spaces if you want more substantial changes to the garden, they will also be able to advise on how to preserve or replant. Costs will vary on scope.

Lighting Designer - Architects include fixed lighting specifications for a functional home with typical lighting controls. If there is an ambition for a more bespoke system, a lighting designer would need appointed. Costs will vary on scope.

MEP or AV Consultant - Architects include specifications for a functional home with layouts, coordination and sizing for straightforward servicing proposals. For complex systems, like use of an heat pumps, MVHR, solar panels or audio/ visual, we recommend a specialist M&E or AV consultant. Costs will vary on scope.

Asbestos Survey - If there is suspected asbestos (based on the period of house and any previous works) a survey is required. Allow £500.

Value Added Tax (VAT)

VAT applies to construction costs in the UK, but the rate varies significantly (0%, 5%, or 20%) depending on the type of project, the property's use, and whether the builder is VAT-registered. The standard rate of 20% is the default for most work unless specific conditions for a reduced or zero rate are met. 

Standard Rate (20%)

The standard rate applies to most construction services and materials, including: 

• Renovations, maintenance, and repairs of existing residential properties.
• Construction of most new commercial buildings (offices, shops, warehouses).
• Building extensions or conservatories.
• Professional services such as architects' and surveyors' fees.
• Hiring goods/equipment. 

Reduced Rate (5%)

A reduced rate of 5% may apply to specific residential projects: 

• Renovating empty homes: For residential properties that have not been lived in for at least two years.
• Conversions: Changing the number of dwellings in a building or converting a non-residential building into a home.
• Energy-saving materials: Installation of qualifying energy-saving materials like insulation, solar panels, and heat pumps.

Zero Rate (0%)

Zero-rated projects mean no VAT is charged. This typically applies to: 

• New dwellings: The construction of a qualifying new residential building, provided specific conditions are met.
• Charity buildings: Construction of new buildings intended solely for non-business charitable use.
• Relevant residential buildings: New buildings used for specific residential institutions like care homes, student halls, or hospices.
• Works for disabled people: Certain alterations to a home to suit a disabled person's condition. 

For detailed official guidance, refer to HMRC's Buildings and construction VAT Notice 708. 



Cost management throughout a project and role of Architects vs Quantity Surveyor

As a RIBA Chartered Practice and as a registered architect through the governing body Architects Registration Board (ARB), we are bound by two codes of professional conduct. The "RIBA Code of Professional Conduct" (link here to full document) states that:

"Members must use reasonable endeavours to advise their client on the need for specialist advice to assist the client to ascertain and control the cost of the project".

It is essential to note that while architects assist clients with managing project costs and can advise from a design perspective on how to strategically apportion budget by prioritising value gains, architects are not professional cost consultants - this is the role of Quantity Surveyors.

If throughout the project you wanted to get a better understanding of costs we recommend employing a Quantity Surveyor (QS). A QS will provide detailed cost plans at the end of every stage and can provide a Bill of Quantities for the Principle Contractor to price during tender. For larger projects (over £500,000), a QS would be an essential appointment to have a dedicated professional manage your budget in detail.

For projects under £500,000, a full QS service may be considered too expensive. However, for a more accurate understanding of costs early on, you can employ a Quantity Surveyor to produce an Initial Cost Plan after the initial design stages. Allow £1,000 - £1,500 for this.

How do we manage cost at Armstrong Campbell Architects?

We believe in an active collaborative approach to design and construction is required to deliver an efficient project for all parties. We work with clients from our first meeting to establish budgets and project scopes. We have a network of collaborators of other professional services and contractors that can provide early engagement to assist with cost planning.

Our Stage 04 tender packages include detailed 'Schedule of Works' which are priced by the contactor for a transparent and itemised tender price. This means prior to construction, costs can be aligned. From this point, the best advice that can be given for managing cost is to avoid changes during construction. Late design changes significantly delays contractors progress on site and therefore usually cost more than the same specification or detail, if included prior to tender.

Extension Costs Overview in London (2026)

Single-storey

The single-storey rear extension remains the most popular domestic project in London. A basic specification includes standard brickwork matching existing, double-glazed aluminium windows and doors, flat roof with membrane waterproofing, standard kitchen fit-out as a separate budget, engineered wood flooring, radiator heating, and standard electrical and lighting. A higher specification may include quality facing brick, large format sliding doors spanning 3-4m, large skylights, underfloor heating, flooring such as microcement or solid wood and high-performance insulation materials.

Two-storey

Two-storey extensions cost more per square metre than single-storey work, which surprises some people initially. The reality is that building upwards introduces complexity at every level. The cost premium exists because of more complex structure with first-floor joists and loadbearing requirements, scaffolding required for height access, two sets of windows and doors, plumbing to first floor often for ensuite bathrooms, more complex roof structure, and longer construction time.

Side returns

Side returns reclaim what was often dead space—a narrow passage alongside Victorian and Edwardian terraces that served only to provide space and light between neighbours. They usually cost more per square metre because of fixed costs (scaffolding, skips, fees etc) spread over fewer square metres as well as structural work removing large sections of existing wall.

Loft conversions

Loft conversions offer better value per square metre than ground floor extensions because you're working within the existing footprint with no foundations needed, smaller scale roof alterations, less external work, and shorter construction time of typically 8-12 weeks.

There is still regulatory complexity as loft conversions trigger fire safety requirements that ground floor extensions don't. You need a protected escape route, which often means upgrading your staircase and installing fire doors throughout the house.

Existing joists are often not structurally robust enough to take the load of a new living floor, requiring additional support. If headroom is generous, a cost effective method can be achieved by retaining existing joists and adding new floor joists in between that are usually deeper. However, typically in Victorian and Edwardian properties the floor to ceiling height is limited in lofts. Given that roof lines are not allowed to be raised due to planning policy, often the required method is to replace the existing floor joists and ceiling to the upper floors.

Additional Considerations for Cost Budgeting

Contingency: 5-10%

Essential buffer for unexpected ground conditions, poor soil, or hidden obstructions, price increases on materials, minor design changes during construction, unforeseen structural issues, and weather delays requiring extended hire. Most projects use 50-80% of contingency and having it in place means these issues become manageable problems rather than project-threatening crises.

Temporary Living Costs

Extensions disrupt daily life more than people anticipate. You can live through a rear extension build, but prepare for dust to penetrate everywhere despite plastic sheeting, for noise starting early and continuing all day, access to garden disappearing, kitchen being unusable for months. Most families cope, but it's tiring and stressful. Allow cost of a temporary kitchen set-up and potentially temporary furniture or fixtures.

If works are extensive, you may need to move out. Should this be the case allow for rental and moving costs. Usually this will be paired with temporary storage facilities for your personal belongings and items such as furniture or curtains that want to remain dust and damage free.

Existing House Works

Extensions don't exist in isolation. Connecting new to old creates interfaces that need resolving. These integration costs add up quickly and are difficult to avoid if you want the finished result to feel cohesive. Following our above Cost Benchmarking Strategy will ensure appropriate budget is given to existing spaces.

Landscaping & External Works

Once you have completed your beautiful new home, you will want a garden that is at least clean and tidy. Gardens are damaged by construction and it is worth budgeting for making good, as it always gets added. Depending on size and scope, it could cost anywhere from £7,500 - £15,000 for a fairly simply reinstated garden. Typical costs to consider:

• Paving adjacent to an extension - £3,000 - £7,000
• Turf and planting reinstatement - £1,000 - £2,500
• Boundary walls or fencing - £1,500 - £4,000
• Drainage and soakaways - £1,000 - £2,500
• External lighting - £500 - £1,500

Frequently Asked Questions About Extension Costs

How accurate are online cost calculators?

Not very. They use average costs per m² that don't account for your specific site constraints, specification preferences, property type and age, location within London, or current market conditions. Use them for very rough guidance with ±30% accuracy, but get proper quotes for your actual project. Online calculators are useful for initial ballpark figures but dangerous for detailed budgeting. Our Cost Benchmarking Strategy provides a means of adding some nuance to an initial estimate, although it should still only be considered rough guidance.

Should I get multiple quotes for professional fees too?

Yes, but don't choose on price alone. Architects charging 6% instead of 10% either aren't including full service, with hidden extras later, or are not allowing sufficient working hours to properly execute the project. Architectural fees reflect service scope and expertise, reducing fees usually means reducing service, which creates gaps that cause problems later.

For other professional services we would typically send out invitations to receive three quotes for the package. We find quotes for surveys and building control services are often very closely aligned. This makes competitive tendering with these packs less essential and sometimes speed of appointment may be prioritised. Architects can advise if single quotes fall within a reasonable and expected range.

Can I reduce costs by managing the project myself?

Theoretically yes but practically, most DIY project managers make expensive decisions without technical expertise, are not experienced enough to catch site issues or manage contractor disputes effectively, and struggle coordinating trades and inspections. This leads to projects overrunning on time, budget and quality.

How much should I budget for a new kitchen?

Kitchen fit-out can cost anywhere from £10,000 - £40,000 depending on size, material quality and fittings. Kitchens are typically the single biggest item spend on projects but are also usually the most-used room in extended homes; compromising excessively here can undermines the overall project.

Budget realistically based on priorities, options include:

• High street kitchens from Howdens, Wickes or Ikea provide good variations of designs and are very cost effective.
• Traditional designer kitchen companies such as Baumstark or Plain English Kitchens are high quality and carry a high price tag.
• A number of new kitchen manufacturers such as Reform or Vermland offer interesting materials and detailing in contemporary designs.
• Commonly on DIY projects, Ikea or Howdens base units are paired with bespoke door fronts from Plykea or Naked Kitchens. While there is cost savings in using basic base units, the door fronts from these companies are premium priced and becomes relatively expensive.
• Finally, contractors we use all have their own joiners or trusted subcontractors that can build bespoke kitchens for surprisingly good value. Plywood is typically used for the base units which is more robust than high street brands chipboard bases. Door fronts can then range from a cost efficient spray painted MDF, to mid-ranged timber veneered plywood, or high-end solid timber or metal finishes.

Is it cheaper to extend upwards (loft) or outwards (rear extension)?

Generally loft conversions cost less per m² than rear extensions (£2,000 - £3,000 vs. £3,000 - £4,000) because no foundations are needed, there is a smaller footprint of altered structure, and less external work. Loft conversions might cost less to build but add less value because additional bedrooms matter less in already-adequate houses. Extensions transforming inadequate kitchens into family spaces often deliver stronger returns despite higher construction costs. However, larger living spaces usually merit the additional bedroom and visa versa, with properties that are extended both out and up, often carrying premiums.

Does Brexit still affect my extension cost?

Material costs have increased 25-35% since 2019 due to Brexit trade barriers, COVID supply disruptions, Ukraine war affecting energy and materials, and general inflation. Currently (2026), costs are stabilising but unlikely to decrease significantly. Budget based on current pricing and include contingency for any further increases during your project.

Should I wait for costs to come down?

Construction inflation continues above general inflation with timber, steel, and manufactured products like windows having all seen sustained price increases. Additionally, energy cost increases affects production costs of materials like tiles and bricks which are highly energy intensive products to make. As these pressures persist, delays to projects become expensive.

Contact us for Full Architecture Design Services, Planning Applications or to book a Home Design Consultation.

Read more of our journal guidance:

What to expect from working with an architect - Do I Need an Architect For A London Extension Or Loft Conversation? →

A detail dive into the design and construction process - Guide To Building Control, Party Wall Agreements and Principle Designer Role. →

Everything you need to know on planning permission - Guide to Planning Permission & Permitted Development in London →

armstrong campbell architects

Based in Leytonstone, London
Residential and Hospitality Architects

House Extension | Loft Conversions | Renovations | New Builds