Site Surveys Explained: What Happens Before a Single Pane of Glass Is Ordered
The foundation of every successful glass extension—why precision matters and what a thorough survey reveals about your project.
Quick Answer
A site survey takes 2-4 hours and captures everything needed to manufacture your glass extension with precision. The surveyor measures your property to the millimetre, assesses ground conditions and drainage, locates services, evaluates structural connections, analyses orientation for thermal comfort, and documents everything with photographs.
This survey prevents problems: foundations designed for your specific ground, drainage routes planned around existing pipes, frames manufactured to fit perfectly, and potential issues identified before they become expensive surprises. Skip the survey (or rush it), and problems multiply.
What Gets Surveyed (And Why It Matters)
- Dimensions: Every measurement to the millimetre—frames are manufactured to fit exactly
- Ground levels: Determines floor heights, step thresholds, and drainage falls
- Soil conditions: Clay, sand, rock—dictates foundation design and depth
- Tree proximity: Roots affect ground movement; may require deeper foundations
- Drainage: Existing drain locations, depths, and routes for rainwater
- Services: Gas, electric, water positions to avoid during construction
- Structural: Wall construction, lintel positions, load-bearing elements
- Orientation: Sun path analysis for glazing specification and comfort
Why the Survey Matters More Than You Think
A glass extension isn’t assembled from standard parts pulled from a warehouse. Every frame, every glass panel, every roofing component is manufactured specifically for your project—cut and fabricated to dimensions unique to your property.
Get the measurements wrong, and nothing fits. Miss a drain, and you’re digging it up mid-build. Underestimate ground conditions, and foundations fail. Overlook a structural constraint, and the design doesn’t work.
The site survey is where precision begins. It’s the foundation upon which everything else is built—quite literally. A thorough survey costs time upfront but saves exponentially more in problems, delays, and compromises later.
The Survey vs. The Initial Consultation
These are different things:
- Initial consultation: Discusses your requirements, explores design options, provides indicative pricing. Typically 1-2 hours. May include basic measurements.
- Technical site survey: Captures precise measurements and technical data for manufacturing. Typically 2-4 hours. Happens after you’ve committed to proceed.
Some companies combine these; others separate them. Either way, you need both—the design conversation AND the technical precision.
The Eight Stages of a Thorough Survey
A comprehensive site survey follows a systematic process. Here’s what happens and why each stage matters:
Initial Walkthrough & Discussion
The surveyor reviews the agreed design with you, discusses any changes since the initial consultation, and identifies specific areas requiring attention.
- Design confirmation and refinements
- Access route discussion for construction
- Identification of any concerns or constraints
External Measurements
Precise measurement of the house exterior where the extension connects, plus the full footprint of the proposed structure.
- Wall dimensions and positions
- Window and door locations (heights, widths, reveals)
- Roof lines, fascias, guttering heights
- Extension footprint and boundaries
Ground Level Analysis
Understanding how the ground slopes—both across the site and away from the house. This determines floor levels, threshold heights, and drainage falls.
- Levels at house wall (existing floor height)
- Ground slope across extension area
- Relationship to garden levels
- Step or ramp requirements
Drainage & Services Survey
Locating existing drains, manholes, water pipes, gas lines, and electrical supplies. Essential for avoiding services and planning rainwater drainage.
- Manhole positions and depths
- Drain routes and gradients
- Soil/foul drain identification
- Water, gas, electric locations
Structural Assessment
Evaluating the existing house structure where the extension will connect—wall construction, lintels, load paths, and any constraints.
- Wall construction type (cavity, solid, timber frame)
- Existing lintel positions and sizes
- Load-bearing elements to consider
- Connection details and weathering
Internal Measurements
Measuring connecting rooms—floor-to-ceiling heights, door positions, and internal features that affect the design or installation.
- Ceiling heights and construction
- Internal door dimensions and positions
- Radiator and service locations
- Floor construction and finishes
Photography & Documentation
Comprehensive photographic record of all relevant areas—reference material for the design team, manufacturing, and installation crews.
- All elevations and connection points
- Ground conditions and drainage
- Neighbouring properties and boundaries
- Access routes and site constraints
Design Review & Confirmation
Discussion of any issues identified, design adjustments if needed, and confirmation of the specification before manufacturing begins.
- Review of any site-specific challenges
- Design modifications if required
- Specification confirmation
- Timeline and next steps discussion
What Gets Measured (And How Precisely)
The level of precision required for glass extension manufacturing might surprise you. We’re not working in rough centimetres—we’re working in millimetres.
House Dimensions
Wall lengths, heights, window positions, roof lines—all to ±1mm accuracy
Ground Levels
Slope gradients, threshold heights, drainage falls—to ±5mm accuracy
Drain Depths
Invert levels, connection points, fall requirements—critical for compliance
Window Reveals
Opening sizes, frame depths, cill heights—for perfect frame integration
Roof Angles
Pitch, height at eaves and ridge, fascia details—for roof integration
Boundary Distances
Exact distances to property lines—for planning and Building Regs compliance
Why Millimetres Matter
Glass extension components are manufactured with tolerances of 1-2mm. A frame cut 10mm too long doesn’t “squeeze in”—it doesn’t fit at all. Glass panels are even less forgiving.
Modern surveying uses laser measuring equipment accurate to fractions of a millimetre. Combined with digital recording, this ensures data transfers accurately from site to factory to installation.
Ground Conditions: The Hidden Variable
What lies beneath the surface often determines more about your project than what’s visible above it. Ground conditions dictate foundation design—and foundation design significantly affects cost and complexity.
| Ground Type | Characteristics | Foundation Implications |
|---|---|---|
| Clay | Expands when wet, shrinks when dry. Affected by trees. | Deeper foundations (often 1m+). May need special design near trees. |
| Sand/Gravel | Drains well, stable. Less affected by moisture changes. | Standard depth foundations. Good load-bearing capacity. |
| Chalk | Generally stable but can have voids or soft spots. | Depth depends on chalk quality. Investigation may be needed. |
| Rock | Excellent bearing capacity but difficult to excavate. | Shallow foundations possible. Excavation costs higher. |
| Made Ground | Previously disturbed soil—unpredictable. | May need piled foundations. Investigation essential. |
| High Water Table | Ground water close to surface. | Waterproofing measures. Possible pumping during construction. |
The Tree Factor
Trees near your proposed extension significantly affect foundation design. Tree roots extract moisture from clay soils, causing ground movement. When trees are removed, the soil rehydrates and can heave upward.
The surveyor notes:
- Tree species: Different trees have different root zones
- Distance from extension: Closer trees = greater impact
- Mature height: Influences root zone extent
- Recent removal: Stumps or recently felled trees affect soil
Building Control uses NHBC guidelines to determine foundation depth based on these factors. In high-shrinkage clay with mature trees nearby, foundations might need to be 2.5m deep or more.
Drainage: Planning the Water Routes
Every glass extension needs to deal with rainwater—from the roof and from the area where your patio used to be. The survey identifies existing drainage and plans new connections.
What the Surveyor Looks For
- Existing drain positions: Where are manholes and their depths?
- Drain types: Surface water (rainwater) vs foul (sewage)—they’re separate systems
- Connection points: Where can new rainwater drainage connect?
- Fall requirements: Drains need gradient; is there enough?
- Obstacles: Will new drains need to cross or divert existing ones?
- Soakaways: If connection isn’t possible, can water soak into the ground?
The Manhole Investigation
Surveyors typically lift manhole covers to check:
- Invert depth (the bottom of the drain)
- Direction of flow
- Whether it’s surface water or foul
- Condition of existing drainage
- Available connection points
This information is critical for Building Regulations approval and avoiding expensive mid-build discoveries.
Structural Assessment: Where Extension Meets House
The junction between your existing house and the new extension is critical. The surveyor assesses the existing structure to determine how the extension will connect safely and weathertight.
Key Structural Considerations
- Wall construction: Cavity wall, solid brick, timber frame, or other? This affects fixing methods and weatherproofing details.
- Lintel positions: Where are the existing lintels over windows and doors? New openings may be constrained by these.
- Load paths: If creating a large opening to the extension, how do loads transfer around it?
- First floor: What’s above the connection point? Affects structural options.
- Existing extensions: If connecting to a previous addition, how was that built?
This assessment determines whether structural alterations are needed—new steel beams, padstones, or other support—and feeds into the Building Regulations submission.
Orientation & Solar Analysis
Understanding how your site relates to the sun’s path throughout the day and year is essential for specifying glazing and ensuring year-round comfort.
What Orientation Reveals
- South-facing roofs: Maximum solar gain—requires high-performance solar control glazing
- West-facing walls: Intense afternoon/evening sun—consider shading options
- North-facing glass: Minimal direct sun—standard glazing may suffice
- East-facing: Morning sun—generally manageable
The surveyor documents orientation and any shading from neighbouring buildings, trees, or your own house. This information determines glazing specification—particularly the solar control properties needed to prevent overheating.
Why This Matters for Comfort
A south-facing glass roof without proper solar control glazing can reach 40°C+ on sunny days—unusable. The right glazing specification, determined by understanding orientation, keeps the space comfortable year-round. Temperature-controlled glass is often essential for south and west-facing roofs.
Preparing for Your Site Survey
A little preparation helps the survey run smoothly and ensures nothing is missed.
Before the Surveyor Arrives:
After the Survey: What Happens Next
The survey data flows into the next stages of your project:
Technical Drawings Produced
Detailed CAD drawings showing exactly how your extension fits your property, with all dimensions confirmed.
Typically 1-2 weeksFinal Specification Confirmed
Any adjustments identified during the survey are incorporated. You receive the final specification for approval.
Included in aboveBuilding Regulations Submission
Technical drawings submitted to Building Control or Approved Inspector for approval.
4-6 weeks for approvalManufacturing Begins
Once approved and final payment stage reached, components are manufactured to your survey dimensions.
4-6 weeks typically
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About This Article
This guide draws on 50+ years of Room Outside’s experience conducting site surveys across Kent, Surrey, Sussex, Hampshire, and London. The process described reflects our approach refined over thousands of successful projects.
Last updated: March 2026 | Author: Room Outside Technical Team
Frequently Asked Questions
What happens during a conservatory site survey?
A comprehensive survey takes 2-4 hours covering: precise measurements of house and extension area, ground level analysis, drainage assessment, structural evaluation, service locations, orientation analysis, and photography. The surveyor also discusses design refinements.
How long does a site survey take?
Typically 2-4 hours depending on complexity. Simple conservatories on straightforward sites may be quicker; complex orangeries, sloping sites, or properties with access challenges take longer.
Do I need to be home for the site survey?
Yes. The surveyor needs internal access and your input is valuable for discussing requirements, reviewing the design, and identifying any concerns. Plan to be available throughout.
What should I prepare before a site survey?
Clear the extension area, locate manholes, know where your services are (stopcock, fuse box), have property documents ready, and think about construction access routes.
Why do surveyors check ground conditions?
Ground conditions determine foundation design. Clay requires deeper foundations than sand. Trees nearby affect depth requirements. High water tables need special solutions. Understanding this prevents structural problems.
What measurements are taken during a site survey?
External house dimensions, window/door positions, ground levels, drain locations and depths, roof heights, internal room measurements, boundary distances—all typically to ±1mm accuracy.
Is the site survey free?
Policies vary. Some companies offer free surveys; others charge £150-£500, often refundable if you proceed. Ask upfront about costs.
What happens after the site survey?
Technical drawings are produced (1-2 weeks), final specification confirmed, Building Regulations submitted (4-6 weeks approval), then manufacturing begins. You receive detailed drawings showing exactly how the extension fits.
Call us anytime – David, our digital assistant, will take a few details so the right specialist can follow up personally. 01243 538999 or request a consultation online
Ready to Start Your Project?
Every successful glass extension begins with a thorough understanding of your site. Our experienced team combines precision surveying with creative design to ensure your project is built on solid foundations—literally and figuratively.
Book Free ConsultationCall us anytime on 01243 538999 – David, our digital assistant, will take a few details so the right specialist can follow up personally.
Room Outside: Glass extension specialists since 1973.
Serving Kent, Surrey, Sussex, Hampshire, London & surrounding areas.
