Basement conversion

Important points to consider when considering waterproofing your basement/cellar.

Olympic ConstructionOlympic Construction News

Conversion of basements/cellars is becoming much more popular, as the demand for more usable space, within our homes rises.

Converting the basement/cellar offers a wide variety of options, including additional living/ bedroom rooms, kitchen/dining room, home cinemas, gym area, the area can be designed to suit your needs.

Basement conversion

Important considerations

  1. Choosing the right designer and contractor who understands your needs, the limitations of your property and is suitably qualified, having the knowledge and experience to correctly advise you on the waterproofing design and your legal responsibilities.


(BS:8102:2022 Section 4.1 General Design Philosophy)  

Strategies for dealing with all external sources of groundwater, surface/flood water, soil gases and contaminants should be determined from the very earliest stages of the planning and design processes for any project involving below ground structures.

(BS:8102:2022 Section 4.2 Design team)   

  • be suitably qualified and experienced, commensurate with the type and size of the proposed project;
  • be capable of devising solutions that accommodate the various project constraints and needs with an understanding of construction forms and sequencing; and
  • provide the design team with information and guidance that assists with and influences the design, installation and future maintenance of the waterproofed structure.

The object of waterproofing the basement/cellar is to provide a dry sustainable environment fit for purpose. To achieve this objective, the designer must undertake the following points.

A detailed desktop survey will provide invaluable information to enable the designer to provide his risk assessment, the following information should be gathered and included within the report/specification submitted.

  • The flood risk to the area, including both the risk of flooding water courses and surface water runoff.
  • The ground surrounding the property provides information on the permeability of the soil.
  • Radon/ ground gas risk, including landfill sites within the area.
  • A drainage surveydefective drains present a risk of a head of water coming to bear in close proximity to the property. Hidden drainage system under the property, suitability of the drainage system should your project include connecting to the drainage system.
  • Trial holes should be excavated within the basement, this providing information on the foundation type, depth and groundwater levels.

Planning permission and building regulation.


In most cases planning permission to convert a basement/cellar will not be required, the exceptions being listed below.

  1. Excavating to create a new basement/cellar.
  2. If your property is listed, you will require planning permission.
  3. Installing new openings within the external elevations, may require planning permission.

Building regulations application must be submitted when converting a basement/cellar. 

  • Change of use.
  • Site Preparation and resistance to moisture (Part C)
  • Structure - Walls and foundations (Part A)
  • Fire Safety (Part B)
  • Conservation of fuel and power (Part L1)
  • Ventilation (Part F)
  • Resistance to the passage of sound (Part E)
  • Drainage and waste disposal (Part H)
  • Heat producing appliances (Part J)
  • Access and facilities for disabled persons (Part M)
  • Vehicle access
  • Electrical works (Part P)

Party wall agreement – do you require one ?

Partywall Act
  • The Party Wall etc. Act 1996 was introduced to prevent and resolve disputes in relation to party walls and boundaries shared with your neighbour.
  • It is additional legislation to planning permission and building regulations.
  • Section 6 of the party wall act informs if you are excavating within 3m of an adjoining property and to a greater depth than the foundations of the neighbouring property, which is the case when installing most pump chambers, you must serve a party wall notice on the adjoining property owner/occupiers.
  • The Party wall Act, is very specific to individual developments, a Party Structure Notice must be served if the party wall needs to be underpinned, or you will be cutting into a party wall to insert supporting beams.

The system designs

Once the surveyor has gathered all the information listed above, it will give them a better understanding of the groundwater distribution through the ground and how water under hydrostatic pressure will come to bear against the building walls and floors.

Water can penetrate a structure below ground level in two ways:

    1. Where the ground is not saturated, water will move through the capillaries of the ground and structure by capillary action. When this form of moisture comes into with a building structure, the structure becomes damp, although there will be no free flow (flooding) of water. Capillary moisture can enter a structure laterally, such as when a wall is earth retaining, resulting in salts forming on the internal surface or spoiling of the internal decoration.
    2. Where the ground is saturated due to prolonged rainfall or surface water flooding, water under hydrostatic pressure occurs within the ground, resulting in groundwater being pushed through the capillaries of the ground and structure. Where this situation occurs, the pressure will come to bear against the walls and floors, forcing groundwater into the property in the form of liquid water, and flooding will occur.

Producing a dry basement/cellar

Producing a dry basement requires two distinct and separate steps.

The managed application of waterproof membranes to the internal face of the walls/floors, or by fitting a drainage membrane, which safely manages ingress water to a suitable rate to a collection and disposal point.

Cementitious Membranes systems

Cementitious membranes are modified renders and slurries applied to the internal/external face of the structure, being a crystallising material, they prevent water passing through the structure by forming a plug within the capillaries of the masonry, resisting a hydrostatic water pressure of 195PSI.

As cementitious membranes stop water at the point of entry, pressure can build up behind the membrane, so it is important that the substrate is capable of accepting the induced stresses.

Cavity Drain Membrane systems

Cavity drain membranes are polyethylene sheets with moulded dimples which create a cavity between the membrane and the substrate. Controlling and directing water entering through the structure via set channelling to a collection point which it can be removed by an automatic pump or an atmospheric drainage point.

As cavity drain membranes do not stop water at the point of entry, so do not increase stresses in the substrate, which can happen with cementitious systems, allowing waterproofing to weaker structures.

Hybrid system

A hybrid system can be installed to gain the advantages of both and overcome disadvantages of one or other of the methods on a given project, however this requires a sound knowledge of how water pressure affect the structure of a building and the limitations of the installed materials.


When it comes to advice on waterproofing your basement/cellar, who are you going to call?

Olympic Construction Chartered Building Engineers and Registered PCA Waterproofing Designers
Manchester/ Lancashire 0161-633-9860 | Cheshire/ Derbyshire 01625-900-800