Author Bio:
This article was contributed by Amico Design Architecture, a Northampton-based architectural practice providing design, planning, surveying and technical support for residential and commercial projects. The team works with homeowners on extensions from early feasibility and measured surveys through to planning applications, technical design and visualisation.
Homeowners often focus on the finished result: a larger kitchen, another bedroom or a brighter living space. Less attention may be given to planning restrictions, budget, technical constraints and how the new area will connect with the existing home.
Those early decisions matter. Poor preparation can lead to planning delays, expensive redesigns or an extension that adds space without improving the way the property works.
Here are some of the most common mistakes homeowners make before extending their homes and how they can be avoided.
Starting with size instead of purpose
One of the first questions homeowners ask is usually: “How large can the extension be?”
It is understandable, but it is not always the best place to begin.
A more useful question is: “What problem does the extension need to solve?”
The existing kitchen may feel too enclosed. The family may need another bedroom. There may be poor access to the garden or nowhere suitable to work from home.
Without a clear purpose, homeowners can spend a significant amount of money adding floor space that does not improve the home.
Before deciding on size, consider:
- which parts of the current layout do not work
- which rooms are underused
- whether the home lacks light or storage
- how the family’s needs may change
- whether existing spaces could be reorganised
In some properties, a modest extension combined with internal changes will work better than a much larger addition.
Assuming planning permission is not required
Some house extensions can be completed under permitted development rights, but that does not mean every project is automatically allowed.
Permitted development is subject to limits covering height, position, depth and the relationship between the extension and the original house. Rights may also be restricted in conservation areas, on listed properties or where previous planning conditions apply.
Earlier alterations can also affect what is possible. An extension added by a former owner may already have used part of the property’s permitted development allowance.
Homeowners should check whether:
- the property is listed
- it is in a conservation area
- an Article 4 direction applies
- earlier planning conditions removed certain rights
- prior approval is required
- previous extensions affect the available allowance
The planning position should be checked before a final design is commissioned. Otherwise, time may be spent developing a scheme that is unlikely to proceed.
Confusing planning permission with building regulations
Planning permission and building regulations approval are not the same.
Planning is mainly concerned with whether a development is acceptable in its location. It considers factors such as appearance, scale, local character and the effect on neighbouring properties.
Building regulations deal with how the extension is designed and constructed. They cover matters including:
- structural safety
- insulation
- ventilation
- drainage
- fire safety
- accessibility
An extension may not require a full planning application but can still need building regulations approval.
Homeowners who confuse the two may discover that important technical work has not been completed when they are ready to appoint a builder.
Designing the extension without considering the existing home
An extension should feel like a natural part of the property, not an unrelated room added to the back or side.
Homeowners sometimes focus so heavily on the new area that they overlook what will happen to the existing rooms.
An extension can unintentionally:
- make an existing room darker
- create an awkward route through the house
- remove useful wall space
- leave a room trapped in the centre of the layout
- reduce the connection between the home and garden
The relationship between old and new should be considered from the beginning.
Experienced house extension architects can assess the whole property rather than treating the extension as an isolated addition. This can help homeowners understand how the new layout, circulation, natural light and external appearance will work together.
Prioritising appearance over everyday use
Inspiration images can be useful, but they should not become the only basis for a design.
A large glazed kitchen may look impressive, but homeowners still need to consider privacy, glare, furniture placement, heat and storage.
An open-plan room may feel spacious, but it also needs to accommodate cooking, dining, relaxing and movement without becoming cluttered.
Before approving a layout, it is worth imagining an ordinary weekday rather than a staged photograph.
Ask questions such as:
- Where will coats and shoes be stored?
- Can people move through the room easily?
- Is there enough wall space for furniture?
- Will noise travel through the whole ground floor?
- Can the space adapt as the family changes?
Good architecture should look appealing, but it must also work in practice.
Setting a budget without defining what it covers
Another common mistake is choosing a headline budget without working out what must be included.
Construction is only one part of the cost. Homeowners may also need to allow for:
- architectural and surveying fees
- planning and building control charges
- structural engineering
- drainage investigations
- kitchens and bathrooms
- flooring and decoration
- landscaping
- temporary accommodation
- contingency funds
Quotes can also vary because contractors may exclude different items or use provisional allowances.
Two prices that look very different may not cover the same work.
The clearer the drawings and specification are, the easier it becomes to compare quotes properly and understand what is included.
Leaving cost discussions too late
There is little value in developing a detailed design that is far beyond the available budget.
Cost should be part of the conversation from the beginning. This does not mean reducing every idea to the cheapest option. It means deciding where the budget will have the greatest impact.
For example, a homeowner may prioritise natural light and layout over maximum floor area. Another may choose better insulation and energy performance instead of expensive decorative finishes.
Early cost awareness allows the design to develop realistically and reduces the risk of major changes later.
Underestimating the effect on neighbours
A homeowner sees an extension from inside their own property. A neighbour is more likely to experience it as a change to privacy, daylight or outlook.
Planning officers may consider whether a proposal causes:
- overlooking
- loss of daylight
- an overbearing appearance
- additional noise
- privacy concerns
- excessive building close to a boundary
A neighbour objection does not automatically lead to refusal, but a valid planning concern may carry weight.
Many issues can be addressed during the design stage. Windows can be repositioned, upper floors stepped back and roof forms adjusted.
Considering neighbours early can help reduce objections and improve the overall design.
Relying on inaccurate property information
Estate agent plans and online maps are useful references, but they are not substitutes for a proper measured survey.
An accurate survey records the size and arrangement of the property and its immediate surroundings. Depending on the project, it may also identify:
- boundaries
- drainage positions
- roof forms
- ground levels
- nearby buildings
- important site features
If the existing information is inaccurate, proposed drawings may be based on false assumptions.
Even a small difference in measurements can affect room sizes, structural work or the position of the extension in relation to a boundary.
A proper survey gives the design a reliable starting point.
Ignoring technical constraints until later
A design may look straightforward until questions arise about structure, drainage or access.
Removing an external wall may require substantial structural support. Building near a sewer can introduce further requirements. Limited access can affect how machinery and materials reach the site.
Likely constraints should be identified early enough to influence the design.
Depending on the project, advice may be needed from:
- an architectural professional
- a structural engineer
- building control
- a drainage specialist
- a party wall surveyor
- a utility provider
Finding these issues before construction is usually far less disruptive than discovering them once work has started.
Selecting a builder based on price alone
The cheapest quote is not always the best value.
A low price may mean certain elements have been excluded, underestimated or left as provisional sums. It may also reflect differences in materials, supervision or the contractor’s understanding of the project.
Homeowners should compare quotations on a like-for-like basis.
Useful questions include:
- Has each contractor priced the same drawings?
- Are materials and finishes clearly identified?
- What is excluded?
- Which amounts are provisional?
- How will changes be agreed?
- Who will supervise the project?
- Can the contractor provide recent references?
Detailed drawings and specifications make it easier for builders to price accurately and for homeowners to compare offers fairly.
Starting construction before the design is resolved
Planning drawings are not always detailed enough to build from.
The contractor may still need technical information covering structure, insulation, drainage, materials and construction details. Building regulations approval may also need to be in place.
Starting too early can mean important decisions are made on site under pressure.
That can lead to:
- delays
- unexpected costs
- inconsistent workmanship
- compromises in the design
- disputes over what was included
A little more preparation before construction often prevents much larger problems later.
What should homeowners do before extending?
Before committing to an extension, homeowners should prepare a clear brief.
They should identify what the extension needs to achieve, how much they can realistically spend and which features matter most.
The next steps should include:
- arranging an accurate survey
- checking the planning position
- identifying obvious site constraints
- developing a design around the whole home
- reviewing cost during the design process
- preparing technical information
- comparing contractors carefully
The aim should not simply be to make the property larger. It should be to create a home that functions better.
Amico Design, a team of architects in Northampton, explains:
“The strongest extension projects usually begin with a clear understanding of what is not working in the existing home. Once that has been identified, the design can focus on creating the right space rather than simply the largest possible addition.”
Frequently asked questions
Do all house extensions need planning permission?
No. Some can be completed under permitted development rights, provided they meet all relevant conditions. However, the property’s location, history and previous alterations can affect what is allowed.
Do extensions need building regulations approval?
Most extensions require building regulations approval, even if planning permission is not needed. This covers areas such as structure, insulation, ventilation and fire safety.
When should an architect become involved?
Ideally, professional advice should be sought before the size and layout have been fixed. Early input allows the design to respond to planning rules, budget, technical constraints and the needs of the household.
What is the first step when planning an extension?
The first step is to define what the extension needs to achieve. Homeowners should identify the problems with the current property, their budget and the features that matter most.
Better preparation leads to better extensions
Many extension mistakes begin with a missed restriction, an unrealistic budget or a design that has not been tested against everyday life.
Taking time to assess the existing property, understand the planning position and develop the design properly creates a stronger project from the outset.
A successful extension should provide more than extra floor space. It should improve the way the whole home looks, feels and functions for years to come.