Design Review Before Building: How to Sense-Check Your Extension Plans Before Work Starts

**Why a design review before building matters**:

Most planning drawings for London extensions and loft conversions are prepared to a design-level standard — showing the layout, form, and external appearance of the proposal sufficiently for planning purposes. They are NOT typically prepared to the level of detail required for construction — they do not specify exact wall build-ups, structural connections, drainage routes, or insulation strategies. The gap between planning-approval drawings and construction-ready information is the design review gap — and it is where the most common and costly problems arise.

• A formal design review before construction starts bridges this gap by:
• •Identifying information that is missing or unresolved before work starts
• •Checking that the design as drawn is constructible and does not contain conflicts between different systems
• •Confirming that the design complies with Building Regulations at a detailed level (not just at a high level)
• •Identifying specification decisions that will affect cost and programme
• •Creating a record of the agreed design basis before work starts

**The 10 key areas of a pre-construction design review for a London rear extension**:

*1. Floor levels and finished floor level (FFL)*: Confirm the finished floor level (FFL) of the new extension relative to: the existing house floor level (is there a step up or down? where does the threshold sit?); external ground level (is there a level threshold? what is the fall from FFL to external?); the drainage invert level (if drainage runs beneath the floor, is the drain below the slab with adequate cover?). Mismatches in floor level between the extension and the existing house are one of the most common design oversights — the planning drawing shows both at the same level, but the existing floor is actually at a different level than assumed.

*2. Drainage coordination*: Where does the kitchen sink waste go? Where does the surface water from the flat roof drain? Is there a sewer build-over issue (is the extension built over or within 3m of a public sewer — if so, Thames Water Build Over Agreement may be required)? Confirm drain routes, falls, inspection chamber positions, and connection points. Drainage is frequently underdeveloped in planning drawings and creates surprises on site when excavation reveals existing drainage where it was not expected.

*3. Structural coordination*: Confirm the structural engineer's details have been issued and are coordinated with the architectural drawings. Key checks: beam position and depth (does the beam fit within the ceiling zone without reducing headroom?); padstone positions and sizes (are they shown on the architectural drawing?); column positions (do they conflict with the internal layout?); new floor joist span and depth (is there adequate space in the floor zone for the structural floor?); lintels over all new openings.

*4. Head height and ceiling height*: Verify head height in all areas, including: height to underside of new beam; height in the new extension under a flat roof (checking Part L insulation build-up, structural deck, and ceiling finish all fit within the available zone); height on the staircase (minimum 2.0m at pitch line; 1.9m under any obstruction); height in the loft conversion bedroom (2.0m average minimum; 1.5m over 50% of floor area).

*5. Window and door positions*: Check every window and door opening in the design for: compliance with Part K (window guard heights for low-level windows); overlooking of neighbouring gardens or windows (are any windows that face a neighbouring private space obscure-glazed?); conformity with the planning approval (approved window positions and sizes match the design); bi-fold or sliding door track position relative to finished floor level and any steps or thresholds.

*6. Insulation strategy and Part L compliance*: Confirm the insulation strategy for each element: wall build-up (cavity, outer leaf, inner leaf, insulation type and thickness — Part L U-value ≤ 0.28 W/m²K); flat roof insulation (PIR thickness — Part L U-value ≤ 0.15 W/m²K, typically 150mm PIR minimum); floor insulation (PIR thickness — Part L U-value ≤ 0.22 W/m²K, typically 100mm PIR minimum); glazing specification (U-value ≤ 1.6 W/m²K). The 25% glazing limit under Part L (new glazing area must not exceed 25% of the new floor area, unless an SAP calculation demonstrates overall compliance) — has this been checked?

*7. Roof drainage and rainwater goods*: Where does the flat roof drain? Single outlet or multiple outlets? Is there an overflow provision (secondary drainage outlet in case the primary is blocked)? Where do rainwater downpipes discharge — to the existing rainwater drainage, to a soakaway, or to a new connection? Confirm positions do not conflict with external ground treatment (paving, planters, boundary walls).

*8. Build-up at party wall and external wall junctions*: At the point where the new extension wall meets the existing party wall (or the neighbour's boundary wall), confirm the wall build-up, DPC arrangement, and weatherproofing. The party wall junction is a common source of cold bridges and future dampness if not correctly detailed.

*9. Services first-fix positions*: Before construction starts, confirm the positions of all first-fix services: socket positions; light switch positions; consumer unit upgrade (if required); boiler flue extension (if the boiler is being relocated); UFH manifold position and pump station location; data/TV points. Services that are not confirmed before plastering lead to expensive chase and make-good works after first-fix is complete.

*10. External ground treatment and boundary finishes*: Confirm the external ground-level treatment around the extension: paving specification; drainage channels and gullies; any retaining walls (if the extension is on a sloped plot); boundary fence or wall treatment; relationship of external patio level to internal FFL (level threshold or step?).

**The most common design mistakes that cause problems on site in London extensions**:

*Mistake 1 — Staircase that does not fit*:

Staircases for loft conversions and basement conversions frequently cause problems on site when it becomes apparent that the staircase as drawn does not actually fit within the available space — particularly where it conflicts with existing structural beams, the floor joists, a window position, or the available headroom. The planning drawing shows a staircase in plan form but does not show the detailed 3D relationship between the stair, the floor openings at both levels, and the headroom throughout.

Pre-construction check: produce a detailed section drawing through the stair that shows: the floor opening size at both levels; the headroom at every step (minimum 2.0m at pitch line; minimum 1.9m anywhere); the relation to any beam, window, or structural element above; the handrail and balustrade positions. Confirm with the stair manufacturer (if bespoke) or stair supplier (if stock) that the specific stair will fit in the available opening.

*Mistake 2 — Flat roof drainage detail not resolved*:

Flat roofs that drain towards the party wall and rely on the party wall or the neighbour's roof for drainage are frequently detailed inadequately in planning drawings. On site, finding that the drainage outlet conflicts with a party wall condition, a neighbour's drainage arrangement, or an existing downpipe creates programme disruption.

Pre-construction check: detail the exact roof drainage arrangement: outlet position on the flat roof (rear face or parapet edge?); downpipe route (external surface of rear wall? within a parapet box?); connection at ground level (to an existing rainwater trap/gully, or new); secondary overflow provision.

*Mistake 3 — Beam depth conflicts with ceiling height*:

A steel beam installed over a structural opening between the existing house and the new extension has a specific depth (e.g., a 203mm deep universal beam) that must fit within the ceiling zone above the opening. Where the planning drawing shows a flat ceiling at a specific height throughout, and the structural engineer specifies a beam that is deeper than the ceiling zone allows, one of them must change — either the ceiling drops to accommodate the beam, or the structural engineer redesigns the beam (which may involve a larger or costlier section).

Pre-construction check: once the SE's beam specification is known, check the beam depth + bearing details against the ceiling zone shown in the planning drawing. Confirm with both architect and SE that the beam fits without reducing headroom below minimums.

*Mistake 4 — Bi-fold or sliding door threshold creates a trip hazard*:

Bi-fold doors and large sliding doors require a track at the bottom of the opening (for most systems). If the finished floor level inside and the patio level outside are at the same level (a level threshold for Part M compliance), the door track creates a raised threshold that is both a trip hazard and a weathering risk. If the internal floor is higher than the external, an external step is required — which must be coordinated with the threshold channel drain.

Pre-construction check: confirm the door system specification, the track height, the threshold treatment, the FFL inside and outside, and whether a threshold channel drain is included in the drainage specification.

*Mistake 5 — Kitchen extraction route not coordinated*:

The extraction route for a kitchen range cooker or hood (typically a duct from the hood up through the ceiling and either out through the external wall or through the flat roof) must be confirmed before first-fix. Where the duct route conflicts with structural elements (beams, joists), requires an unexpectedly long run, or exits through a position that would affect planning compliance (a duct outlet visible on the front elevation), the extraction route becomes a problem on site.

Pre-construction check: confirm the extraction route from hood position to external outlet, showing the duct dimensions (typically 125–150mm diameter rigid duct), every bend, and the external outlet position.

*Mistake 6 — Conservatories and orangeries at wrong levels*:

Where an extension was designed on planning drawings without a detailed survey of the existing rear ground levels, the extension drawings may show flat ground on all sides — when in reality the rear garden has a slope, a step, or a raised patio area. On site, excavating for the extension foundation and finding that the foundation depth must be greater than assumed (to reach consistent bearing soil), or that the external patio level will not match the extension threshold as drawn, is a common and disruptive discovery.

Pre-construction check: conduct a detailed topographic survey of the rear garden (levels at multiple points) before finalising the extension design. Confirm external ground levels are correctly shown on the detailed design and that the patio and threshold levels are consistent with the proposed FFL and the existing house floor level.

*Mistake 7 — Existing drainage discovered in unexpected position*:

Excavating for new extension foundations in a London Victorian garden and discovering an existing drainage run (foul water, surface water, or a combined sewer) in the path of the new foundations or beneath the extension footprint is a frequent construction site problem. This requires design changes: the drain must either be diverted (an additional cost and time), built over (Thames Water Build Over Agreement required and engineered foundation design needed), or the foundation design adapted.

Pre-construction check: conduct a CCTV drainage survey of the existing drainage at the property (and check Thames Water's sewer map for public sewers) before finalising the extension design. Identify all existing drainage runs within and around the extension footprint.

**Who should be involved in a pre-construction design review for a London extension?**

*The architect or designer*: Responsible for the architectural design drawings (floor plans, elevations, sections, specification). Should be involved in resolving any conflicts identified in the review and in producing the detailed construction drawings needed for Building Regulations and construction.

*The structural engineer*: Responsible for the structural design (foundation, beam, floor structure). Should confirm that all structural elements are coordinated with the architectural design — beam sizes, padstones, lintels, floor joist sizing.

*The principal contractor*: Responsible for sequencing, construction methodology, and programme. The contractor's input in the design review identifies practical buildability issues: access constraints, sequence conflicts, material lead times that must be ordered before construction starts (structural steel 3–5 weeks; bi-fold doors 4–12 weeks; bespoke timber windows 6–12 weeks; clay roof tiles up to 8 weeks).

*The client (homeowner)*: Responsible for confirming the design intent and making decisions where the review identifies choices (e.g., whether to drop the ceiling slightly to accommodate the beam, or invest in a smaller structural section to maintain ceiling height). The client should be present at or engaged with the design review process.

**How to formalise a design review**:

• For a straightforward single-storey rear extension, a design review can be as simple as:
• •A single joint meeting between the architect, structural engineer, and principal contractor, held before construction starts
• •An agreed set of construction drawings (not just planning drawings) that incorporate the SE's structural details, the drainage layout, and the insulation specification
• •A specification document confirming the build-up of each element (walls, roof, floor, glazing) and the material specification for each
• •A pre-construction checklist confirming all the key points above have been confirmed and resolved

For larger or more complex projects (basement, loft conversion, large extension), a more formal design coordination process (sometimes called a 'design freeze' meeting) should be held, with minutes circulated and agreed actions with named responsible parties and deadlines.

**The value of the design review in a design-and-build arrangement**:

Where the client has appointed RCB Design & Build as a design-and-build principal contractor (rather than using a separate architect and then tendering to contractors), the design review process is integrated into the design-and-build process. The principal contractor is involved from the early design stage — coordinating structural, drainage, insulation, and services design as part of the pre-construction phase rather than as a separate review exercise. This integration is one of the key advantages of the design-and-build approach: design problems are identified and resolved before they become site problems, rather than being discovered when the concrete is already poured.