Trimble Architects

Where do I start when building an extension? We look at the process of building an extension as well as question to ask before you start the project.

Where Do I Start When Building An Extension?

If you are asking “extension where to start”, this guide walks you through the key stages from the first idea to completion. It explains what to check, who to speak to, and how to manage costs so your extension project adds value and works for your family. 

DO I NEED PLANNING PERMISSION FOR AN EXTENSION?

Set your goals and scope

Begin by writing a simple brief. Explain why you want a home extension, what problems it will solve, and how you live now. Think about living space, storage, and the way rooms connect. A kitchen extension with open plan dining can change daily life, but so can a modest rear extension that makes space for a desk or utility. List the rooms you need, the size you expect in metres, and any windows and doors that matter to you, for example large glazing to the garden. 

Decide early whether the project is a single storey, two storey extension, loft conversion or garage conversion. The type and size will affect costs, permissions and the time it takes to build. Your brief will help you and any professionals stay on the same line from the start.

Understand planning permission and permitted development

Building regulations and building control

All house extensions must comply with building regulations. Building control checks structure, fire safety, insulation, ventilation, drainage, and electrics. You may use your council’s building control team or an approved inspector. In both cases you will obtain staged inspections and a completion certificate at the end. Without this, selling later can be hard and lenders may refuse lending on the property. 

You may need a structural engineer to design beams, foundations and any new openings in load-bearing walls. If you are removing a chimney breast, adding large bi-folds or changing roof structure, expect calculations and drawings. Building control will ask for these to ensure the structure is safe.

Talk to your neighbours and consider the Party Wall

If you share walls or are excavating near a boundary, the Party Wall etc. Act may apply. A party wall agreement can be simple if everyone agrees, but allow time. Speak to your neighbours early and show them your plans. Good communication gives peace of mind and reduces issues during noisy works.

Check services, access and site setup

Think about drains, manholes, gas, water and electrics. Moving services can add cost. Check where materials can be stored and how trades will reach the rear of the house. If access is through the home, you will need protection to keep dust down and your existing property safe. Plan how you will live on site, for example a temporary kitchen if the project affects cooking space.

Funding, fees and insurance

Speak to your lender if you plan to remortgage or take further borrowing. Budget for professional fees, planning fees, building control fees and any structural engineer costs. Ask your insurance provider if your policy covers building work and if you need to increase the rebuild cost. Your main contractor and any subcontractors should hold public liability and, where relevant, professional indemnity insurance.

Design, drawings and materials

Good drawings save money and time. An architect or architectural technologist can design layouts, elevations and details that meet your needs and the regulations. For a simple single storey extension you may still benefit from professional drawings to coordinate structure, drainage and services. Discuss materials early, for example brick to match the existing property, render, cladding, roof tiles, flat roof systems or roof lights. The choice of materials affects appearance, cost and lead time. 

If you want open plan space, consider how to zone it with light, floor levels and furniture rather than lots of walls. Position windows and doors to reduce heat loss and maximise daylight. Think about the impact of height and eaves on neighbours and on your own garden.

Choosing your team

You can manage the project yourself, hire an architect to lead, or appoint a design and build company. For larger projects, a main contractor handling trades can be worth the cost. Ask friends for recommendations, check recent projects in your area, and read reviews. Visit at least one completed job to see the quality. 

When comparing builders, give each the same drawings and specification so the quotes are like for like. Check lead times, stage payments, and what happens if you make changes. A clear contract will set out scope, programme, how variations are priced, and how completion is agreed and signed off. This clarity reduces risk for all parties.

Programme and key stages

Start planning the schedule before you begin. A typical programme follows this order: design and surveys, planning permission or permitted development checks, building regulations drawings and structural design, tender and quotes, contract signing, site set-up, foundations, walls and structure, roof, windows and doors, first fix services, plastering, second fix, finishes, snagging, and completion. Weather, trades and supply chains can affect time. Build in float for delays so you can manage expectations.

Managing change and staying on budget

Changes on site are the main reason budgets rise. Set your scope and stick to it where possible. If you want to add items, agree the cost in advance and get it in writing. Keep a list of decisions with dates, for example kitchen layout, sanitary ware, flooring and paint. This helps your team order on time and keeps the project moving. 

A quantity surveyor can help with cost control on larger builds. For smaller projects, a simple spreadsheet and regular checks against the contract sum will ensure you stay in control.

Neighbour and street impact

Construction affects people around you. Agree delivery times, keep access clear, and make sure the site is tidy. Noise rules may apply through your local authority. Good manners reduce complaints and keep the project friendly.

Sustainability and comfort

Better insulation, airtightness and sensible glazing improve comfort and reduce bills. Ask your team about options for higher performance materials, recycled content, low-water fittings and efficient heating controls. Think about cross-ventilation and shading to avoid summer overheating. Small choices during design can have a higher impact than expensive tech later.

Common mistakes to avoid

People often begin without clear drawings, skip early research on rules, or forget to allow for fees and VAT. Others do not set aside a contingency or fail to check what is included in quotes. Rushing to begin can lead to higher costs and a longer programme. Take time to find the right professionals and contractors for your project and location.

How Trimble Architects can help

Here at Trimble Architects, we guide you from the first idea to the last sign-off. We can help you check whether you need planning permission, prepare drawings for building regulations, speak to building control, and coordinate structural engineers and other trades. We can also support you when you apply to your local planning authority, manage party wall matters with neighbours, and find a reliable builder. If you want design advice on kitchen layouts, open plan living or windows and doors, our team can help you choose options that suit your home and budget. 

If you would like ongoing tips about house extensions, rules and costs, you can join our newsletter for simple updates.

Designing an extension is a process, not a single step. If you take time to research, find the right team, and set a clear plan, you can extend your home with confidence and achieve a result that feels part of the original house while meeting your needs now and in the future.