An extension is one of the most rewarding ways to improve a home, but it is also one of the easiest projects to get wrong. An architect does far more than draw pretty pictures: they interrogate how you actually live, test what your budget will realistically buy, steer the scheme through planning and building control, and stay involved until the last skirting board is fixed. This guide explains what that process looks like from the inside.

What an Architect Actually Does

Most people come to an architect with a rough idea — a bigger kitchen, a garden room, a loft-like space at the back of the house. The first job is to turn that instinct into a brief that can be costed and built. Good architects ask awkward questions early: how do you use the space at seven in the morning and at nine at night? Do you need a utility room more than a formal dining area? Where does the sun land in June?

From there, the work usually follows a staged process:

• Feasibility and concept — measured survey, options, sketch plans and an early indication of cost.
• Developed design — planning drawings, materials, and the point at which the design is largely fixed.
• Technical design — construction drawings, specifications, structural coordination and building regulations submissions.
• Construction stage — tendering, appointing a builder, administering the contract and inspecting the work on site.

Fees are usually quoted either as a percentage of the build cost or as a fixed sum per stage, so you can pause between stages if the numbers stop working.

Brief, Budget and Feasibility

Be honest about money before a single line is drawn. Build costs vary enormously across the UK, and your architect should be able to give you a realistic range per square metre for your area and specification. Always hold back a contingency of at least 10 to 15 per cent — older houses have a habit of revealing surprises once floors come up.

Feasibility also means understanding the constraints of your plot. A surveyor or architect will check levels, drainage runs, boundary positions, tree roots, and whether the property sits in a conservation area or is listed. Restrictions such as rights of light, easements or a shared party wall can shape a scheme more than any design preference, so it pays to uncover them at the start rather than halfway through construction.

Planning Permission and Building Regulations

Many extensions fall under permitted development rights, meaning you may not need a full planning application. There are limits on depth, height, eaves, materials and how much of the garden the extension covers, and these differ between houses, bungalows and flats. A lawful development certificate is worth obtaining even when you believe you are covered — it is a small cost that protects you when you come to sell.

Larger schemes, homes in conservation areas, listed buildings and properties affected by Article 4 directions will need a full application. This is where an architect earns their keep, preparing drawings, a design and access statement and a heritage justification that respond directly to local policy.

Planning permission and building regulations are separate approvals. Building control focuses on safety and performance: structural calculations, fire resistance, ventilation, thermal efficiency, safe glazing and, increasingly, protection against overheating. Your architect coordinates this information and makes sure the builder is working to an approved set of details.

Coordinating the Wider Team

A typical extension involves a structural engineer, a party wall surveyor, a building control body and the main contractor. The structural engineer will size steel beams, padstones and lintels, and may also advise on foundations if ground conditions are poor. Under CDM 2015, someone must take on the principal designer role, and an architect is well placed to do so.

When it comes to builders, you can either tender competitively or negotiate with a contractor you trust. Either way, a written contract matters. A JCT contract sets out payment terms, variations, insurance and completion dates, and having your architect administer it — valuing work, issuing instructions, certifying payments — keeps the relationship professional rather than personal.

Interior Design: Joining Old and New

The best extensions are designed from the inside out. Start with the layout of the rooms you use most: where the sofa faces, how far the kitchen island sits from the sink, whether a boot room can absorb coats and muddy boots. Ceiling heights, bulkheads and the depth of roof structures all affect how generous a space feels, and a good architect will resolve these before they are built rather than afterwards.

Thresholds are where extensions succeed or fail. A continuous floor level and one flooring material running through the opening makes the new space feel like part of the house; a step and a change of level makes it feel like an add-on. Light is equally important — rooflights and roof lanterns draw daylight deep into a plan, while carefully positioned windows frame views and manage solar gain in summer.

Finally, consider the quiet details: built-in joinery, a decent utility, somewhere to charge devices, lighting circuits that create different moods. These cost relatively little during construction and are difficult and expensive to add later.

Keeping the Project on Track

Extensions rarely fail because of design; they fail through drift. Decisions get postponed, variations creep in, and the budget quietly erodes. Retaining your architect through the build stage gives you someone whose only loyalty is to the design and the agreed sum, not to the quickest way of finishing the job.

Expect to be asked to confirm samples, tiles, taps and paint colours at set points, and try to answer promptly. Keep a single folder for drawings, approvals and invoices. When the work finishes, work through the snagging list methodically, collect your completion certificate and warranty documents, and hold the final payment until everything is settled. Do that, and you will end up with a home that works as well as it looks.