Energy Performance Certificates for Commercial & Domestic Buildings

Assess Ltd

Tel        0121 453 8332

Mob     07771 662 357

Email   [email protected]

SAP for Extensions

For new extensions that contain glazing with a combined surface area of more than 25% of the new floor area Building Control you will need to supply them with SAP and U-value Calculations before issuing your completion certificate, these must be produced by an accredited On Construction Energy Assessor (SAP Assessor). (complete our SAP Questionniare)


This includes windows, roof-lights and glazed doors which could cause excessive overheating and (theoretically) require additional energy to maintain normal household temperature. This potential additional energy usage has an effect on the overall emissions rate of the building which needs to comply with Building Regulations Part L1B as does each of the new fabric elements of the extension.

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Building Regulations Part L1B require the new fabric elements of your extension (walls, floor, roof, windows, doors) to achieve at least the minimum u-values specified within tables 1,2 & 3 in addition to the overall Dwelling Emission Rate (DER) being no greater than than the DER of a theoretical version of the property with the permitted glazing area.


Tables 1, 2 and 3 of Building Regulations Part L1B indicate the minimum u-values (thermal effciency) allowed for your new walls, floor, roof, windows and doors (building fabrics), these must be achieved.

We will provide u-value calculations (and advice if necessary) for each of your fabric elements as required by Building Control.



Once your building (including the new extension) has been 'modelled' within our (SAP) software we will produce a Part L1B Complaince Report which will indicate the (proposed) DER for your building.

We will then produce a copy of your building and adjust the new glazing to equal a combined area of no more than 25% of your extension floor area to obtain a new (notional) DER.


We can provide all the documentation that will be required by Planning and Building Control and offer EARLY ADVICE and SPECIFIC RECOMMENDATIONS on any potential non compliance issues so please get in touch as soon as possible.

Architects, Architect, Builder, Contractor, Developer, Self Build, EPC, Building Regulations Part L1B compliance, building regulations part L1B, L1B Extensions, u-values for extensions, extension u values,

In addition to calculating the U-Values for the Fabric of the dwelling we need to consider the efficiency of the HVAC services (Heating, Ventilation & Air Conditioning) and can offer you practical advice in ensuring that these contribute to the overall ratings rather than purely meeting minimum requirements.

Best advice to ensure a compliant and speedy SAP ASSESSMENT


Fill in our SAP Questionnaire THOROUGHLY ... Most delays are caused because of missing information that we need to complete your assessment. Everything we need (pretty much) is requested on our SAP Questionnaire so completing it thoroughly will certainly save time (naturally no need to repeat information already contained on your drawings and specifications).


BE SPECIFIC ... concerning your fabric constructions, we need to understand the type and thickness of each material that you have used and specific insulation types (eg - 'Celotex GA4000' not just 'Celotex')


CONTACT US EARLY ... so we can advise you of potential compliance issues before they become expensive re-works.

It's not only highly glazed EXTENSIONS that require SAP Assessment you will also require a SAP EPC for NEW BUILD HOMES, Barn CONVERSIONS, COMMERCIAL to DOMESTIC conversions and

SINGLE to MULTIPLE dwellings (flats & appartments).


Existing Domestic EPC's and Commercial EPC's are produced using software that relies on many 'assumed' values (RdSAP & SBEM) however there are instances when calculated U-Values can give a far more accurate and representitive calculation for the property.


ONLY SAP ASSESSORS  /  On Construction DEA's

are qualified to produce U-Value calculations.


EPC Assess are quaified and accredited :-


On Construction Domestic Energy Assessors ( OCDEA / SAP ASSESSORS )

Non Domestic Energy Assessors ( NDEA / Commercial Assessors )

Domestic Energy Assessors ( DEA )

Green Deal Assessors - Domestic & Commercial ( GDA / NDGDA )


... so can provide Energy Performance Certificates for (virtually) ANY PROPERTY TYPE and have years of experience working with architects, designers, builders, contractors large and small on thousands of standard and not-so-standard developments across the UK. We are happy to share our knowledge and do what we can to ensure your projects easily acheives compliance however EARLY INVOLVEMENT is CRITICAL if we are to guide you in the most cost effective way.

Opening like windows & doors generally lose more energy than the surrounding building fabric however glazing can provide useful Solar Gains that can be used to compensate for the losses providing they do not cause summer overheating, we can ensure that an optimum balance is achieved to ensure compliance.

It's never too early to get us involved and we are happy to be as involved as you would like us to be and hope our experience a cross a range of building types will allow us to be of service no matter what your next project entails.




  ... so we can help you with what you have planned for tomorrow !


Please contact us on the form below and let us know more about your project, you can attach drawings if you would like specific advice or information and we'll be happy to call you back at your convenience :-


In order to achieve compliance with Building Regulations Part L1B there are two seperate 'targets' to acheive, individual U-value targets for each of your fabric elements and an overall Dwelling Emission Rate target (DER). 


Minimum Fabric U-value are clearly stated in tables 1, 2 and 3 of Building Regulations Part L1B, these are minimum values for walls, floor, roof, windows and doors, these must be achieved and we produce U value Calculations to varify this.

(SAP U-value calculations are only valid if produced by an accredited SAP Assessor you cannot rely upon calculations produced by any other means)


Dwelling Emisson Rate (DER) is the measure of the overall CO2 emissons created by your building. Changes that you make such as adding an extension (which is open to the rest of the property) will affect your DER so there limits applied within the building regulations to ensure the changes are contolled, such as limiting the amount of new glazing that you are allowed to install. However if your DER is calculated across the whole building (existing and new) the affect of any 'over glazing' (over and above the 25% allowed) will be 'shared' over a much larger volume so will only make a very minor difference to the DER.

We will produce Compliance Reports showing the DER of your Proposed building and a Notional (compliant) building to show the DER is no worse for the sake of the additional over-glazing.


We will provide all the documentation that you require for Building Control and liase directly with your BCO to ensure that Part L1B is achieved and he has the information required to issue your completion certificate promptly and without fuss. We will advise you of any potential complaince issues and provide specific options that you can consider to ensure L1B compliance.

However the earlier you contact us within your project the more help we can be in terms of ensuring that you will be compliant while you are still in a position, if need be, to make cost effective changes early.



SCALED DRAWINGS including floor-plans, elevations, sections and site layout for the entire building (new extension and existing building).


HEATING/COOLING specifications of space and water heating systems, ventillation and any low-carbon technologies (LZC), including any new systems that you are going to install as part of your extension project).


CONSTRUCTION SPECIFICATIONS for your new fabric elements - walls, floors, roof, windows and doors. The fabrics of your existing building will be calculated using standardised thermal efficiencies based on its age (or other documentation that can be provided). 


We may also send you our SAP QUESTIONNAIRE to collect any outstanding information that we require (you can send these documents to us electronically or in printed format).


It is important that you provide as much information as possible otherwise we are required to use DEFAULT inputs which tend to be 'worse case scenario' and can contribute to issues of non compliance.


Once we have collected all of your information we will 'model' your data within our software and initially produce a Part L1B Checklist and U-value Calculations to ensure that your building is compliant with the appropriate Building Regulations and calculate a DER for the 'proposed' building.


We will make a copy of your building and adjust the new glazing area down to no more than 25% of the new floor area to obtain a second (notional) DER figure which we can comare with the (proposed) DER.


The DER of the Proposed building should be no higher than the DER of a Notional building which complies with Part L1B

Service and Documents Provided


U-value Reports (for your NEW fabrics)


Part L1B Compliance Report (for your PROPOSED building)

SAP Reports


Part L1B Compliance Report (for your NOTIONAL building)

SAP Reports


Part L1B Assessment Summary Report


SAP EPC & Reports

Orangary, glazed extension, lantern, rooflight, bi fold doors, slifing doors, glazed wall, bifold door, Rear Extension, ground floor extension, patio dooor, house extension,