Converting a Conservatory to a Habitable Room: London Part L Compliance Guide

UNDER BUILDING REGULATIONS APPROVED DOCUMENT L1B (CONSERVATION OF FUEL AND POWER IN EXISTING DWELLINGS), A CONSERVATORY IS EXEMPT FROM THE THERMAL PERFORMANCE REQUIREMENTS THAT APPLY TO EXTENSIONS IF — AND ONLY IF — TWO CONDITIONS ARE MET. FIRST, THE CONSERVATORY MUST BE SEPARATED FROM THE MAIN HOUSE BY WALLS, DOORS, OR WINDOWS THAT MEET THE RELEVANT THERMAL STANDARDS (TYPICALLY THE SAME WALLS, DOORS, OR WINDOWS THAT EXISTED BEFORE THE CONSERVATORY WAS BUILT). SECOND, THE CONSERVATORY MUST BE CAPABLE OF BEING MAINTAINED AT AN AMBIENT TEMPERATURE INDEPENDENT OF THE MAIN DWELLING — IN PRACTICE, THIS MEANS IT SHOULD NOT HAVE A CENTRAL HEATING RADIATOR OR OTHER PERMANENT HEAT SOURCE CONNECTED TO THE HOUSE'S HEATING SYSTEM.

IF EITHER CONDITION FAILS — IF THE WALL BETWEEN THE CONSERVATORY AND THE HOUSE IS REMOVED TO OPEN THE SPACES, OR IF A RADIATOR IS INSTALLED FROM THE HOUSE SYSTEM — THE CONSERVATORY CEASES TO BE AN EXEMPT STRUCTURE AND BECOMES PART OF THE REGULATED THERMAL ENVELOPE OF THE DWELLING. AT THAT POINT BUILDING REGULATIONS REQUIRE THE CONSERVATORY TO MEET THE SAME THERMAL STANDARDS AS A NEW EXTENSION: WALLS AT 0.28 W/M'K OR BETTER, ROOF AT 0.18 W/M'K, FLOOR AT 0.22 W/M'K, AND WINDOWS/DOORS AT A MINIMUM 1.4 W/M'K WHOLE-WINDOW U-VALUE.

THIS IS THE CRITICAL TRIGGER MOST HOMEOWNERS AND EVEN SOME BUILDERS ARE UNAWARE OF. A TYPICAL CONSERVATORY WITH A POLYCARBONATE OR THIN GLASS ROOF HAS A ROOF U-VALUE OF 1.8 TO 3.5 W/M'K — FOUR TO TEN TIMES WORSE THAN THE REQUIRED 0.18 W/M'K. SIMPLY REPLACING THE POLYCARBONATE WITH GLASS AND ADDING A RADIATOR WITHOUT ADDRESSING THERMAL PERFORMANCE IS A BUILDING REGULATIONS BREACH AND WILL BE FLAGGED ON THE EPC AND CONVEYANCING SEARCHES.

MOST CONSERVATORY-TO-ROOM CONVERSIONS DO NOT REQUIRE PLANNING PERMISSION BECAUSE THE FOOTPRINT AND EXTERNAL APPEARANCE OF THE STRUCTURE ARE UNCHANGED. UNDER THE GENERAL PERMITTED DEVELOPMENT ORDER 2015 (AS AMENDED), ALTERATIONS TO THE ROOF COVERING AND THERMAL UPGRADES TO WALLS AND GLAZING DO NOT TYPICALLY CONSTITUTE DEVELOPMENT REQUIRING PERMISSION.

HOWEVER, PLANNING PERMISSION MAY BE REQUIRED IN THE FOLLOWING CIRCUMSTANCES. IF THE CONSERVATORY WAS ITSELF BUILT UNDER PERMITTED DEVELOPMENT AND YOU ARE NOW MAKING THE INTERNAL CONNECTION TO THE HOUSE PERMANENT (BY REMOVING THE ORIGINAL WALL), THIS COMBINED DEVELOPMENT MAY NEED RETROSPECTIVE ASSESSMENT. IF THE PROPERTY IS IN A CONSERVATION AREA AND THE CONSERVATORY IS VISIBLE FROM THE PUBLIC HIGHWAY, PERMITTED DEVELOPMENT RIGHTS FOR ROOF ALTERATIONS ARE RESTRICTED UNDER ARTICLE 4 DIRECTIONS IN MANY LONDON BOROUGHS. IF THE PROPERTY IS LISTED, ANY EXTERNAL ALTERATION — INCLUDING ROOF REPLACEMENT — REQUIRES LISTED BUILDING CONSENT REGARDLESS OF SIZE.

RCB'S STANDARD ADVICE IS TO SUBMIT A HOUSEHOLDER PRE-APPLICATION ENQUIRY TO THE LOCAL AUTHORITY BEFORE COMMITTING TO ANY ROOF REPLACEMENT WORKS ON A CONSERVATORY IN A CONSERVATION AREA OR LISTED SETTING. THE FEE IS TYPICALLY £50 TO £100 AND PROVIDES WRITTEN CONFIRMATION THAT THE WORKS ARE PERMITTED OR IDENTIFIES WHAT CONSENT IS NEEDED.

THE MOST EFFECTIVE ROUTE TO PART L COMPLIANCE FOR THE CONSERVATORY ROOF IS REPLACEMENT WITH A SOLID TILED WARM ROOF SYSTEM. THESE SYSTEMS REPLACE THE GLAZED OR POLYCARBONATE ROOF SHEETS WITH A LIGHTWEIGHT INSULATED STRUCTURAL PANEL THAT CAN BE TILED TO MATCH THE MAIN HOUSE ROOF.

THE LEKA SYSTEM (ULTRAFRAME) IS THE MARKET LEADER. THE CORE PANEL IS A STRUCTURAL INSULATED PANEL (SIP) OF 100MM OR 150MM GRAPHITE-ENHANCED EPS WITH TIMBER FACING. THE SYSTEM IS FIXED DIRECTLY TO THE EXISTING CONSERVATORY ROOF STRUCTURE — NO ADDITIONAL STRUCTURAL STEELWORK IS TYPICALLY REQUIRED UNLESS SPANS EXCEED 4M. THE DECLARED THERMAL PERFORMANCE OF THE LEKA SYSTEM AT 100MM IS 0.16 W/M'K — COMFORTABLY BELOW THE PART L 0.18 W/M'K LIMIT. TILES CAN BE MARLEY MODERN, REDLAND STONEWOLD, OR PLAIN CONCRETE TILES DEPENDING ON THE EXISTING ROOF SPECIFICATION.

SUPALITE TILED ROOF SYSTEM BY CELSIUS IS AN ALTERNATIVE THAT USES A PROPRIETARY RAFTER SYSTEM WITH KINGSPAN KOOLTHERM K107 PITCHED ROOF BOARD AT 70MM (0.023 W/MK) ACHIEVING 0.17 W/M'K. SUPALITE IS HEAVIER THAN LEKA AND MAY REQUIRE STRUCTURAL ASSESSMENT OF THE EXISTING CONSERVATORY UPRIGHTS AND BASE IF THE ORIGINAL STRUCTURE WAS NOT DESIGNED FOR A SOLID ROOF LOAD.

EQUINOX TILED ROOF SYSTEM BY ULTRAFRAME USES A DIFFERENT PANEL CONFIGURATION SUITED TO LEAN-TO CONSERVATORY PROFILES. IT ACHIEVES 0.15 W/M'K AT THE STANDARD PANEL DEPTH AND INTEGRATES SPECIFICALLY WITH ULTRAFRAME ROOF SYSTEMS.

ALL THREE SYSTEMS INCLUDE A PLASTISOL-COATED STEEL INTERNAL LINER THAT FORMS THE CEILING FINISH — THESE COME IN WHITE OR CREAM AND PROVIDE A CLEAN INTERIOR WITHOUT ADDITIONAL PLASTERBOARD. DOWNLIGHTERS CAN BE INCORPORATED IN THE LINER SYSTEM.

WHERE THE CLIENT WISHES TO RETAIN A GLAZED ROOF BUT IMPROVE THERMAL AND SOLAR PERFORMANCE, THE OPTION IS TO UPGRADE THE GLAZING SPECIFICATION RATHER THAN REPLACE WITH A SOLID ROOF. THIS IS A VALID PATH FOR CERTAIN CONTEMPORARY CONSERVATORY STYLES BUT CARRIES A SIGNIFICANT LIMITATION: NO COMMERCIALLY AVAILABLE GLASS ROOF SYSTEM ACHIEVES A U-VALUE CLOSE TO THE 0.18 W/M'K REQUIRED BY PART L FOR THE ROOF ELEMENT.

THE BEST-IN-CLASS TRIPLE-GLAZED ROOF GLASS — PILKINGTON OPTITHERM S3 PLUS IN TRIPLE CONFIGURATION — ACHIEVES A CENTRE-PANE U-VALUE OF APPROXIMATELY 0.6 W/M'K AND A WHOLE-UNIT U-VALUE (INCLUDING FRAME AND EDGE SPACER LOSSES) OF APPROXIMATELY 0.8 TO 1.0 W/M'K. THIS IS STILL FIVE TIMES WORSE THAN THE PART L OPAQUE ROOF REQUIREMENT.

HOWEVER, UNDER A COMPENSATORY APPROACH USING APPROVED DOCUMENT L1B'S TRADE-OFF METHODOLOGY, IF THE FLOOR, WALLS, AND GLAZED ELEVATIONS EXCEED PART L REQUIREMENTS BY SUFFICIENT MARGIN, AND IF WHOLE-BUILDING SAP ASSESSMENT DEMONSTRATES THAT THE REGULATED ENERGY CONSUMPTION OF THE DWELLING DOES NOT WORSEN, THE GLAZED ROOF MAY BE ACCEPTABLE TO BUILDING CONTROL. THIS APPROACH REQUIRES A SAP ASSESSOR TO PRODUCE A COMPLIANT ASSESSMENT PRIOR TO BUILDING CONTROL APPROVAL.

FOR SOLAR CONTROL IN SUMMER, PILKINGTON ACTIV BLUE OR SAINT-GOBAIN COOL-LITE XTREME COATINGS REDUCE SOLAR HEAT GAIN COEFFICIENT (SHGC) FROM APPROXIMATELY 0.6 FOR CLEAR GLASS TO 0.3 TO 0.4, DRAMATICALLY REDUCING OVERHEATING. SELF-CLEANING EXTERNAL COATING (PILKINGTON ACTIV SELF-CLEAN) REDUCES MAINTENANCE ON AN OVERHEAD ROOF GLASS INSTALLATION.

CONVERTING A CONSERVATORY TO A HABITABLE ROOM REQUIRES THE WALLS AND FLOOR TO MEET PART L STANDARDS AS WELL AS THE ROOF. CONSERVATORY WALLS ARE TYPICALLY ONE OF THREE TYPES: DWARF BRICK WALLS (450MM TO 900MM HIGH) WITH SINGLE OR DOUBLE GLAZED PANELS ABOVE; FULL-HEIGHT BRICKWORK; OR FULL-HEIGHT GLAZED ALUMINIUM OR UPVC FRAMES.

DWARF WALLS IN OLDER CONSERVATORIES ARE COMMONLY SINGLE-SKIN 102MM BRICK WITH NO CAVITY AND NO INSULATION — U-VALUE APPROXIMATELY 2.0 W/M'K. BRINGING THESE TO PART L STANDARD REQUIRES INTERNAL INSULATION: KINGSPAN KOOLTHERM K18 INSULATED PLASTERBOARD COMPOSITE (62.5MM TOTAL = 45MM PIR + 12.5MM PLASTERBOARD, 0.022 W/MK) FIXED WITH DRY-LINING ADHESIVE DABS AGAINST THE INNER FACE OF THE BRICK WALL. THIS ACHIEVES APPROXIMATELY 0.30 W/M'K — SLIGHTLY ABOVE THE 0.28 W/M'K TARGET BUT ACCEPTABLE UNDER THE REASONABLE MAXIMUM IMPROVEMENT PROVISION OF PART L WHERE INTERNAL SPACE IS LIMITED.

FOR THE FLOOR, CONSERVATORIES ARE TYPICALLY BUILT ON A SOLID CONCRETE SLAB AT THE SAME LEVEL AS THE GARDEN. WHERE THE FLOOR LEVEL ALIGNS WITH OR IS CLOSE TO THE GROUND FLOOR OF THE HOUSE, THE FLOOR CAN BE UPGRADED WITH 50MM XTRATHERM THIN-R PIR BOARDS AND 65MM CEMFLOOR SCREED TO ACHIEVE 0.22 W/M'K, ADDING APPROXIMATELY 115MM TO THE FLOOR LEVEL. WHERE THE CONSERVATORY FLOOR IS LOWER THAN THE HOUSE FLOOR, THIS ALSO PROVIDES AN OPPORTUNITY TO BRING THE LEVELS CLOSER TOGETHER, REMOVING A POTENTIAL TRIP HAZARD.

ALL WINDOWS AND EXTERNAL DOORS IN THE CONVERTED SPACE MUST MEET THE PART L MINIMUM STANDARD OF 1.4 W/M'K WHOLE-WINDOW U-VALUE. REPLACEMENT WITH A-RATED DOUBLE OR TRIPLE GLAZED UNITS IN UPVC OR THERMALLY BROKEN ALUMINIUM FRAMES IS STANDARD FOR THIS UPGRADE.

A CONSERVATORY-TO-HABITABLE-ROOM CONVERSION ALWAYS REQUIRES BUILDING CONTROL APPROVAL BECAUSE IT INVOLVES A CHANGE TO THE REGULATED THERMAL ENVELOPE OF THE DWELLING. THE APPROPRIATE ROUTE IS FULL PLANS APPLICATION — THIS PROVIDES ADVANCE APPROVAL OF THE DESIGN BEFORE WORKS BEGIN AND PROTECTS THE HOMEOWNER IF THE LOCAL AUTHORITY CHANGES INTERPRETATION OF THE REGULATIONS MID-PROJECT.

THE FULL PLANS APPLICATION SUBMISSION SHOULD INCLUDE: EXISTING AND PROPOSED FLOOR PLANS AND SECTIONS AT 1:50 SCALE; SPECIFICATIONS FOR ALL THERMAL ELEMENTS (ROOF, WALLS, FLOOR, GLAZING) WITH DECLARED U-VALUES AND PRODUCT DATA SHEETS; A SAP ASSESSMENT IF A TRADE-OFF APPROACH IS BEING USED FOR THE ROOF; STRUCTURAL CALCULATIONS FOR THE NEW ROOF SYSTEM (REQUIRED FOR LEKA, SUPALITE, AND EQUINOX SYSTEMS); DRAINAGE DETAILS IF THE FLOOR LEVEL IS BEING ALTERED; AND FIRE SAFETY DETAILS FOR ANY INTERNAL DOORS CONNECTING THE NEW ROOM TO THE MAIN HOUSE — DEPENDING ON THE ESCAPE ROUTE CONFIGURATION, AN FD30S FIRE DOOR (JELD-WEN FD30S SHAKER WITH LORIENT LAS23 SEAL AND PERKO PC100 DOOR CLOSER) MAY BE REQUIRED.

BUILDING CONTROL INSPECTION STAGES FOR A CONSERVATORY CONVERSION TYPICALLY INCLUDE: FOUNDATION/STRUCTURAL BASE INSPECTION BEFORE FLOOR WORKS; INSULATION INSPECTION BEFORE SCREED IS POURED; ROOF STRUCTURE INSPECTION BEFORE ROOF PANELS ARE FIXED; AND FINAL INSPECTION BEFORE COMPLETION CERTIFICATE IS ISSUED. THE COMPLETION CERTIFICATE IS ESSENTIAL FOR FUTURE SALE AND REMORTGAGE.

THE COST OF CONVERTING A CONSERVATORY TO A HABITABLE ROOM IN LONDON VARIES SIGNIFICANTLY WITH SIZE, EXISTING CONDITION, AND SPECIFICATION. INDICATIVE RANGES FOR A STANDARD 3M X 4M (12M') CONSERVATORY ARE AS FOLLOWS.

SOLID TILED ROOF ONLY (LEKA OR SUPALITE SYSTEM, SUPPLY AND FIT): £5,500 TO £9,000. THIS DOES NOT INCLUDE WALL OR FLOOR THERMAL UPGRADES, ELECTRICAL WORK, OR BUILDING CONTROL FEES.

FULL PART L COMPLIANT CONVERSION (SOLID ROOF + INTERNAL WALL INSULATION + FLOOR INSULATION + REPLACEMENT WINDOWS/DOORS + ELECTRICAL ALTERATIONS + PLASTERING + DECORATION): £12,000 TO £25,000 FOR A STANDARD 12M' CONSERVATORY. A LARGER 20M' CONSERVATORY FULL CONVERSION CAN REACH £35,000 TO £45,000.

IF THE WALL BETWEEN THE CONSERVATORY AND THE HOUSE IS BEING REMOVED TO CREATE AN OPEN-PLAN SPACE, ADD A STRUCTURAL STEEL BEAM (TYPICALLY UB 203X133X25 TO UB 305X127X37 DEPENDING ON SPAN AND LOAD) AT £3,000 TO £7,000 INCLUDING STRUCTURAL ENGINEER FEES, PROPPING, PADSTONES, AND MAKING GOOD.

BUILDING CONTROL FULL PLANS APPLICATION FEES WITH A LONDON LOCAL AUTHORITY ARE TYPICALLY £500 TO £900. STRUCTURAL ENGINEERING FEES FOR ROOF LOAD ASSESSMENT AND STEEL BEAM DESIGN ARE TYPICALLY £800 TO £1,500.

TIMESCALE: A CONSERVATORY CONVERSION FROM BUILDING CONTROL APPLICATION TO COMPLETION TYPICALLY TAKES 10 TO 16 WEEKS INCLUDING 6 TO 8 WEEKS FOR BUILDING CONTROL PLAN APPROVAL AND 4 TO 6 WEEKS ON SITE.