A house extension is often the most cost-effective way of creating more space for an expanding family. Moving to a new house can be expensive and a lack of local options makes this a daunting thought.

At Andrew Black Design, we excel in creating innovative solutions to problems with existing houses, whether it is re-configuring a poor layout or extending to create more usable space. From simple kitchen extensions to large glass sun-rooms, we can enhance even the humblest house.

We can take you from concept design stage, through planning and building approvals to helping with a trusted local builder.

Seaforth Drive Extension
Edinburgh home extension folding doors

How we design an extension

We advise our clients, except for the most basic of projects, to employ Andrew Black Design to carry out a full architect service from stages 0 to 5.

The services below (0-4) are generally the minimum you require from your Architect, to cover your legal responsibilities and are suitable for simpler projects for experienced clients only.

How much does our design service cost?

We prefer, where we can, to provide you with a fixed cost for your project.
We have detailed below a generic proposal, based on a bespoke house extension, such as a sun-room, costing in the region of £60,000 excluding VAT.

The complexity of the design and site matters may reduce or indeed increase the fee, so please get in touch to get an accurate quote.

Note – Consultant fees and application costs are excluded and paid direct by the client.


or 13% of the build cost (ex VAT)

Download our Extensions Guide

How much will my house extension cost?

Estimating the build cost of an extension can be a difficult task, with so many factors that influence the cost, including the size, complexity and site access. We can though, lead you through the process to understand the costs better and help you make informed decisions.

How do we estimate cost?

As a starting point we always do our best to work to your budget, so understanding this at the outset is important. Before we start designing, we will try and analyse what you are trying to achieve and work out if this is viable, before we put pen to paper. There isn’t any point spending time and money if the budget and aspirations aren’t compatible.

Once we have worked up initial concept designs, we will look at working out an approximate cost based on square metre rates. This is a fairly blunt way of working out the cost and only really works for simpler projects. A simple house extension with good access for building, on a flat site, might be as little as £2500 per metre squared plus VAT, but rise to say £2750 on a more difficult site. Some of the more architectural projects we have worked on can rise to £3500 or even £4000 per metre square plus VAT.

How do we obtain more accurate costs?

Until the project goes to competitive tender to a list of building contractors, you will only have an approximate cost to work to. To gain more accurate costs we often employ a Quantity Surveyor, who can prepare a Probable Cost for your project. They will analyse each element and make assumptions based on their experience and costs from previous projects. This is a much more accurate way of preparing an estimate for your project. This method is particularly suited to projects where there are internal alterations in addition to the actual extension. Remember that the square metre rate reduces the bigger the project gets due to economies of scale.

Where can I make savings?

The areas where we often see costs rise are where high-end sliding doors and rooflights are specified. We all like the minimal lines of some of the high-end door manufacturers like Maxlight, but these types of doors can cost two or three times other options. If you want that look, go for it, but it could add another £10 to £15K to a build. Expensive kitchens are another budget buster, with some companies charging large mark ups for fairly run of the mill kitchens. Don’t discount using Ikea or Howdens and finishing it all with nice bespoke worktops. Flooring is another area that can mount up so shop around and make sure you have looked at quality as well as price. We can though use our experience to guide you through options.

How much will my extension add to the value of my house?

It is difficult to put a value on good design and a valuation surveyor may not include good design in the value of the property, but a purchaser is more likely to see the true value. Don’t forget it is difficult to value the enjoyment you will get out of an architect designed extension over say 20 years. We have worked on houses that have been very poorly extended in the past and the extension could be argued as reducing the  value of the property. Saving a couple of thousand pounds on design fees may reduce the value of the final product and more importantly the enjoyment of the space.

Frequently asked questions

Extending your home doesn’t have to be complicated and we have detailed below some of the most common questions we are asked.

We have an experienced team of Architects and Architectural Technicians and will be able to answer most of your questions or point you in the right direction.

Detailed Planning Approval

Full planning permission is required before proceeding to Building Warrant Approval. A full application can be made, sometimes referred to as a Reserved Matters Application when following a ‘Planning in Principle’ consent.

Detailed plans, elevations, sections, site plans, location plans and design statement accompany the application, providing enough information to allow the local authority to assess the proposals. All the above is carried out by Andrew Black Design on your behalf as part of Stages 0 to 3 above.

Permitted Development

In some restricted situations you can carry out work without making formal planning approval. Smaller extensions and alterations, especially to the rear of properties can be classed as ‘Permitted Development’, if the build fits a set of conditions in relation to size and distances to boundaries. If the project is ‘Permitted Development’ we would normally apply for a ‘Certificate of Lawfulness’ on your behalf so you have suitable paperwork in place if you sell the property in the future.

Building Standards Approval

An application must be made for new buildings in Scotland under the Building (Scotland) Regulations. The application is made to the relevant local authority and covers details of items such as wall, roof and floor construction, structure, drainage, insulation and heating.

We do this for you, preparing the drawings and specification and making the building standards application for you. We do all the negotiations with the Building Standards Officers and organise the Structural Engineers’ SER certificate for the whole project, which covers the foundations, exterior beams and lintols and the timber structure, including the roof trusses.

Value Added Tax

In most situations, VAT is applicable on house extensions unfortunately. VAT is also charged on our professional fees regardless of the rating of the actual build, in line with all professional services related to building.

SER Structural Certificate

This can only be issued by an Engineer who is an Approved Certifier of Design (Building Structures). This certificate is issued with the application to local authorities for Building Standards Approval and ensures that all aspects of design of the structure of a project satisfy the requirements of the Building (Scotland) Regulations. This will include the kit structure and elements like the foundations, exterior lintols and the staircase.

We will liaise with the chosen Structural Engineer on your behalf.

Who do I get to build my extension?

The actual builder of your extension will be the key to the success of your project. The decision to appoint a contractor should be based on thorough research of potential candidates and should include details of similar, previous projects and references, current workload, suitable insurance cover and key sub-contractors.

A NHBC registered builder or membership of suitable organisations such as Federation of Master Builders is helpful, but relevant referrals are usually the most important. We don’t usually recommend carrying out a building project without any professional inspections, unless you are relatively experienced and have a good knowledge of the contractor.

The most important part of any construction relationship is the contract and agreement between you. Always use a contract, which states the cost, payment frequency, retention sums, what is included and not included and dispute resolution procedures. If you choose to use Andrew Black Design for Stage 5, we will organise the contract on your behalf.

What is ‘Timber Frame Construction ‘?

We normally use tried and tested timber frame technology, in various forms. Engineered to high levels of accuracy and quality, the system allows speedy construction.

Now used in over 75% of new homes in Scotland, the skills are readily available to make the most of this efficient building process. Using timber from managed forests, timber kits are by far the most environmentally friendly way to build.

Some key benefits of timber frame:

  • Significantly simplifies on site construction
  • Allows speedy construction to wind and watertight reducing costs
  • High levels of insulation and air tightness easily integrated
  • Off-site manufacture leads to greater control and quality

Energy Efficient Homes

  • We appreciate that energy use and its resultant cost is a major factor for most self-builders. Further to this, our designs allow integration of many alternative energy saving technologies and offer the option to upgrade insulation levels and specification.
  • Timber kits allow the use of the highest efficiency insulation materials and reflective breather membranes, meaning walls can exceed current building regulation. We also make the most of Accredited Construction Details, issued by the Scottish Government. which, when followed correctly, allow the reduction of air leakage in the building, reducing heating and energy bills substantially.

Preparing energy calculations such as SAP and U-values in house, allows us to design our houses around the whole energy use of the building, rather than just individual elements. We can then take into account the heating, lighting, solar gain and heat loss to ascertain the actual energy use and CO2 created.

Our house extension designs will allow the easy integration of various alternative technologies. Whether you are planning a heat pump from water, ground or air sources, our floor constructions can be altered easily to allow underfloor heating to be used, the most efficient use of heat pump technology.