Structural Designer Cost: Loft Conversions and Home Extensions Explained

If you are planning a loft conversion or home extension, you will need structural calculations and drawings. Many people are unsure what a structural designer actually does and what a fair fee looks like. This guide explains the role of the structural engineer, typical structural designer cost for lofts and extensions, and how to get value for money.

What a Structural Designer Does on a Project

Role on loft conversions

For a loft conversion, a structural designer will usually:

• Check the existing walls and foundations.
• Design the new steel beams, joists and trimmers.
• Produce calculations and drawings for Building Control and your builder.

They make sure the new floor and roof loads are safely carried down into the structure.

Role on home extensions

For extensions, the structural engineer will:

• Check how the new opening affects existing walls.
• Design any new foundations, pads or piles.
• Size the steel beams, posts and roof members.

The aim is a safe, buildable design that fits your architect’s layout.

What you get in a design package

A typical package for domestic work includes:

• Structural calculations for beams, joists and foundations.
• Marked-up drawings or full structural plans.
• Support for Building Control queries.
• Some engineers also include a short structural report where needed.

Structural Designer Cost for Loft Conversions

Typical price ranges

For a straightforward loft conversion in a typical UK house, structural designer cost often falls roughly between £450 and £900 for the structural work, depending on scope.

What affects the fee

You pay more when:

• The loft is large or split into several rooms.
• There are dormers, hip-to-gable changes or big openings.
• Access to information is poor and the engineer must spend more time measuring or checking on site.

What is usually included

Most loft design fees cover:

• Steel beam design and layout.
• Floor joists and trimmer design.
• Connection assumptions and bearing details.

Extra items like checking party wall issues, unusual foundations or site visits may be charged separately.

Structural Designer Cost for Home Extensions

Single storey rear extensions

A simple single storey rear extension often needs:

• One or two steels over a new opening.
• Foundation checks.
• Roof design if the form is non-standard.

Structural designer cost for this type of work is often in the £400 to £800 range, depending on span and complexity.

Side and wrap-around extensions

Side or wrap-around extensions can involve more steelwork and load paths. There may be:

• Corner steels or frames.
• Floor changes and local strengthening.

Fees can move into the £700 to £1,200 range where frames or complex roofs are involved.

Extra steelwork or complexity

You may pay more if:

• There are several load-bearing walls removed.
• The property has previous movement or awkward geometry.
• The architect’s layout pushes spans to the limit and needs more iterations.

Structural Calculations Cost vs Full Design Service

“Calcs only” jobs

Some homeowners only need structural calculations for one RSJ over a wall removal. These are often priced as a fixed fee. Structural calculations cost for a single beam might be in the £250 to £450 range, depending on the scope and the company.

Full drawings and support

A full design service includes:

• Calculations.
• Clear drawings for the builder.
• Follow-up answers to Building Control questions.

This costs more but saves time and stress on site. It can also reduce disputes between contractor and engineer.

Cheap quotes and hidden costs

Very low structural designer cost can mean:

• Limited scope (calcs only, no drawings).
• Extra charges for revisions, site changes or Building Control queries.
• Slow response if the builder runs into an issue.

It is usually better to compare what is included rather than chasing the lowest number.

How to Hire a Residential Structural Engineer the Smart Way

Questions to ask before you instruct

• What exactly is included in your fee?
• Will I get drawings as well as calculations?
• How do you handle revisions if the builder needs small changes?

What a clear scope should include

A good proposal should mention:

• Which parts of the project are included.
• Whether foundations, steelwork and timber design are covered.
• How many revisions or queries are allowed within the fee.

Red flags to avoid

• Vague quotes with no written scope.
• No mention of Building Control.
• No professional indemnity insurance information.

When You Might Pay More Than the Average

Difficult access or complex geometry

Loft conversions in older properties, split-level floors or very irregular layouts need more design time. That pushes fees up.

Previous movement or poor foundations

If the house has cracks, history of subsidence or shallow foundations, more investigation and careful design are needed. This can mean higher structural designer cost.

Last-minute changes during the build

If layouts change on site, extra steels or design checks may be needed. Engineers may charge for additional work beyond the original scope.

FAQs about Structural Designer Cost

Is a more expensive structural engineer always better?

Not always, but extremely low fees often mean limited service. Look for clear scope, experience with domestic work and good communication.

Can I reuse structural calculations from another house?

No. Calculations are project-specific. Different spans, loads and foundations mean you need a fresh design.

Do I still need an architect?

For most lofts and extensions you need both. The architect plans the layout and appearance. The structural engineer makes it stand up.

Can Building Control accept online-only calculations?

Yes, as long as the calculations and drawings are clear, and the engineer is suitably qualified and insured.

Get a Fixed Structural Design Fee for Your Loft or Extension

If you want a clear structural designer cost for your project, you can send your drawings and a short description of the work to SECalcs. We provide fixed-fee structural calculations and drawings for:

• Loft conversions
• Home extensions
• Load-bearing wall removals