Drainage Surveys and Build-Over Agreements

Some extensions affect existing drains or sewers.
 

You may need to consider:
 

• the location of existing drainage;

• whether a sewer runs beneath or close to the extension;

• whether manholes need to be moved;

• whether a CCTV drainage survey is advisable; and

• whether a build-over agreement is required from the relevant water authority.
   

Drainage issues are much easier to deal with before construction starts.
 

A builder discovering an unexpected drain after excavation has begun can result in additional cost, delay and redesign work.

 

Tree Surveys and Arboricultural Advice

Trees can influence the design of an extension, particularly where:
 

• large trees are close to the proposed foundations;

• the property is within a conservation area;

• a tree is protected;

• neighbouring trees may be affected;

• root protection needs to be considered; or

• deeper foundations may be required.
   

A tree survey or arboricultural report may be requested as part of a planning application or may be sensible before detailed construction planning begins.
 

Trees can affect both the planning position and the structural design.

 

Asbestos Surveys

Older homes may contain asbestos-containing materials.
 

This can be relevant where the proposed work involves:
 

• removing an old conservatory;

• altering ceilings;

• removing soffits;

• disturbing textured coatings;

• dismantling outbuildings;

• stripping older floor coverings; or

• carrying out extensive refurbishment work.
   

An asbestos survey may be advisable before demolition or intrusive work begins.
 

The survey cost is normally modest compared with the risk of discovering asbestos unexpectedly after the builder has started work.

 

Specialist Surveys and Reports

Some projects need additional specialist input.
 

Depending on the property and the location, you may need:
 

• a topographical survey;

• a drainage survey;

• a flood-risk assessment;

• an ecology or biodiversity report;

• a bat survey;

• a heritage statement;

• a daylight or sunlight assessment;

• a noise report;

• an energy assessment;

• a soil investigation;

• a utilities search; or

• advice relating to a listed building or conservation area.
   

Most ordinary house extensions will not need all of these.
 

However, unusual sites, larger projects and sensitive locations can require more supporting information.

 

Interior Design, Kitchen Design and Glazing Advice

The extension structure is only part of the project.
 

You may also wish to allow for advice and design work relating to:
 

• kitchen layouts;

• lighting;

• fitted storage;

• flooring;

• interior finishes;

• bifold or sliding doors;

• rooflights;

• roof lanterns;

• bathrooms;

• utility rooms; and

• landscaping.
  
   

A good kitchen layout should be considered early because it may affect windows, doors, drainage, electrics and the overall structure.
 

Similarly, the size and position of glazing can influence the architectural design and structural calculations.

 

Project Management

Some homeowners manage the construction process themselves.
 

Others prefer to appoint a project manager or suitable professional to assist with:
 

• obtaining quotations;

• reviewing the scope of work;

• coordinating contractors;

• answering site queries;

• monitoring progress;

• reviewing variations;

• keeping records;

• checking invoices; and

• helping the project remain organised.
   

Project management adds to the professional budget, but it may provide value where the homeowner does not have the time, confidence or experience to manage a busy construction project.

 

Statutory and Third-Party Fees

Remember that some costs are paid to third parties rather than to your designer or builder.
 

These may include:
 

• planning-application fees;

• lawful-development-certificate fees;

• Building Control charges;

• water-authority fees;

• land-registry documents;

• planning-history searches;

• specialist reports;

• parking suspensions;

• skip permits;

• scaffold licences; and

• other local-authority charges where relevant.
   

The exact requirements depend on the property and the proposed work.

 

How Much Should You Allow?

There is no single figure that applies to every extension.
 

Professional costs depend on:
 

• the size of the extension;

• the type of property;

• the planning route;

• the number of revisions;

• the complexity of the structure;

• the amount of technical detail required;

• whether neighbouring properties are affected;

• whether specialist reports are needed; and

• the level of support you want during construction.
   

A simple rear extension may require a relatively focused professional package.
 

A larger wraparound extension, double-storey addition or complex renovation may need a broader team and a larger allowance.
 

The important point is to include professional costs in your budget from the beginning rather than treating them as unexpected extras later.

 

Do Not Confuse the Cheapest Quote With the Best Value

Professional fees can vary considerably.
 

A lower quotation may look attractive, but check what has actually been included.
 

Ask whether the fee covers:
 

• a measured survey;

• design discussions;

• revisions;

• planning drawings;

• submission of the application;

• technical drawings;

• structural calculations;

• Building Control coordination;

• responses to queries;

• site visits;

• project-management support; and

• VAT where applicable.
   

Two quotations may appear to cover the same service while offering very different levels of detail and support.
 

Clear drawings and good advice can help you make better decisions, compare builder quotations more fairly and reduce the risk of expensive mistakes during construction.

 

Planning a house extension and trying to understand the likely professional costs?

Tell us about your property, the type of extension you are considering and the stage you have reached.

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Where appropriate, APN Extensions may introduce you to relevant independent professionals, consultants and service providers who can help you move the project forward.

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You are under no obligation to appoint any provider introduced through APN.