Basement & Loft Conversions

Loft Conversion

A Loft Conversion involves building an additional room/s such as a bedroom and a bathroom in the loft space of your home. If you are looking to increase the living space in your home without eating into your garden with an extension, then a loft conversion is a cost-effective way to achieve this. A Rear Dormer loft conversion is the most common type of loft conversion that we carry out. This is because so many houses can incorporate them, including a terrace, detached and semi-detached. What Is A Rear Dormer Loft Conversion? Shown below in the diagram is the basic shape of a rear dormer.

rear dormer

They have square, vertical sides and normally have box-like, flat roof. We always mention a flat roof but in-fact they are built with a 1:50 fall to the side to prevent any standing water pooling on top. Rear dormer loft conversions can be built to all different shapes and sizes. From small box dormers that are discreet and stepped in, to large dormers that are built right to the back and sides of the property. The bigger the rear dormer, the more floor space and headroom is achieved on the inside.

What are rear mansards ?

Rear mansards are a great way of converting your loft, especially when certain planning restrictions prevent you from doing a rear dormer. Shown in this diagram is the basic shape of a rear mansard. They have square, vertical sides often built in either brickwork or tile-hung. They have a slopped facing side that slopes bottom in (normally angled inwards between 70˚ and 74˚) to soften the look.

rear mansard

We always mention that both dormers and mansards have flat roofs on their tops. But they are always built with a 1:50 fall to the side to prevent any standing water pooling on top of your flat roof.

Mansards can be built in all different shapes and sizes. From small mansard windows, right through to full-width mansards that are built right across the back of the property.

The bigger the mansard is built, the more floor space and headroom is achieved on the inside. The only difference with a mansard is that the slope across the face of the mansard will make the construction look softer from the outside. It will, however, sacrifice some head height compared to a dormer that is built vertical off the back wall.

Even though we always try and promote a dormer over a mansard, there are circumstances when a mansard is the only feasible option. Situations such as planning permissions, conservation areas, listed buildings and/or your local authority simply not liking dormer conversions in their council policy.

On the plus side, mansards can be built to look amazing and there are many options to choose from. Like the sides can be built in brickwork to match the existing property.

Mansards are often seen to be built up in brick sides to match the existing bricks on the parapet walls. But this can only be achieved through planning permission as you will be building on the party wall.

Other options include choosing to have small dormer windows, or French doors built within the face of the mansard. The options are endless and Mansards really suite the original properties of the Victorian era.

What Is A Hip to Gable?

A hip to gable loft conversion is where the side roof is removed off the side wall. This maximises the internal head height. So, what is a hipped Roof? A hipped roof has three slopped sides see below)

rear mansard

This type of roof is found on many end of terraced, semi-detached and detached properties. This type of conversion requires the builder to remove the hipped part of the roof at the early stages of the process, so the floor steels can be installed, and the gable can be erected.

The gable also allows the floor area within the loft to be extended to the total footprint area. Hip to gable loft conversions are more complicated to construct than a standard rear dormer or mansard.

Hip to gables are slightly more expensive and typically cost 20% more than a standard rear dormer conversion.

The most important factor for undertaking a hip to gable loft conversion is the huge change in the roofline. This will allow you to achieve a massive amount of additional floor space, maximising your loft conversion potential.

The exterior finish can either be tile hung with tiles or slates to match your existing roof, finished in block and painted render or finished in pebble dash if the side of the property is already pebble dashed. The final choice will be dependent on you and discussed on-site to which of these finishes will look most in keeping with your property.

Incorporated in most gable end walls will be a double glazed window to match the existing property style. This provides natural light to either the loft area or the new loft stairwell depending on the final design.


The one thing that most people want in their home is more space, and increasingly homeowners are looking to find this by converting and extending their cellar to create a basement storey beneath their existing property.

Unlike loft space at the top of the house, which lends itself best to creating additional bedrooms, a basement is located close to the main living areas and access, and as such has a more flexible range of uses.

If you want to create (or make use of) space below you home, there are three main routes:

  • Creating a basement as part of a new build
  • Renovating an existing basement or cellar
  • Creating a new basement in an existing home, either directly underneath, or by extending out into the garden, front or side of the house.

A basement is the ideal location for additional family living space, such as a playroom or home entertainment room. It is also a great place to relocate the utility room, boiler and storage, freeing up more valuable above-ground space.

Alternatively, a basement can have its own external entrance and provide a self-contained unit, ideal for use as a home office or annexe. If you want the basement to become an independent separate dwelling (perhaps to sell on), you need express consent.

Planning Permission

Converting an existing cellar beneath a dwelling from a storage area to habitable space does not require planning permission. This will be covered by your home’s Permitted Development (unless you are in a Conservation Area or special designated area, or your home is Listed).

Reducing the floor level of a cellar to improve the ceiling height is, however, treated as an extension and so may need planning permission. Under certain circumstances, however, modest extensions and alterations can be undertaken without the need to make a planning application.

Planning policy on basements varies but it is very difficult for a local authority to find reasonable grounds for refusal, especially if the work does not significantly alter the building’s appearance.

Building Regulations and Basements

The creation of a new habitable basement will require Building Regulations approval regardless of whether it involves a change of use of an existing cellar, or the creation of a new or larger basement through excavation.

The Building Regulations are statutory minimum construction standards that ensure buildings are safe, hygienic and energy efficient. The renovation of an existing habitable basement, or the repair of a cellar that does not involve a change of use, i.e. from storage to storage, is excluded from the Building Regulations.

It is best to make a Full Plans Application for a cellar conversion, rather than to follow the Building Notice procedure, as this allows all design details to be resolved in advance of the work.

Party Wall Act

If the proposed works affect a Party Wall – e.g. if beams are to bear onto a Party Wall; the wall is to be extended, altered, underpinned; or if excavations are to be carried out near to a Party Wall – the owners and leaseholders of both the building within which the proposed basement works are to take place and those of adjoining properties must be informed. The relevant legislation is the Party Wall etc. Act 1996.

JMB can handle planning, Building Regulations approval and any Party Wall agreements as part of our service.

Waterproofing Your Cellar

If you are converting an existing space, you will need to make sure it is dry.

  • Tanking:
  • Waterproofing below ground level is often referred to as ‘tanking’ — the application of a layer of waterproof material directly to the structure. This is usually a cementitious waterproof render system on the walls, typically applied in several layers, linked to a waterproof screed on the floor. Tanking can also involve a sheet membrane, asphalt or other liquid-applied waterproofing material.

    Tanking is also required to withstand the external water pressure around the cellar (hydrostatic pressure). The pressure from the water table around a basement can be enormous and unless the tanking is very securely fixed to the substrate, it can fail. Hydrostatic pressure will force water through the tiniest fault very rapidly and once a leak occurs it can be very difficult to isolate and repair.

  • Cavity Membranes:
  • Cavity drain membranes are an alternative. The membranes are used to create an inner waterproof structure in the basement or cellar, behind which is a cavity (created by the membranes studded profile) that is fully drained, so any tiny leaks in the outer structure are diverted harmlessly away via a drain.

    By constantly draining away any small leaks there is never any water pressure against the inner structure. Several reports consider cavity membranes to be the most reliable way to waterproof a basement.

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