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Architecture & Planning Southampton Ltd
Extensions, New Builds & Licensing


Extensions and New Builds

The ideal start is to meet at the property in question to discuss your intended plans. We can then start to  develop your ideas to find the best design for you, that would also be acceptable to the local planning authorities.

The next step is to measure and draw the existing layout to enable us to begin the detailed design of your project. We aim to get you a preliminary drawing, of the new building, after only a few days, this gives you an opportunity to make any final adjustments and give it your approval. We can then complete the design/drawing stage within 7 days.

At this point, through our careful design and choice of materials, we are able to keep your building costs to a minimum, whilst, at the same time, achieving the full potential from the space you have available.

The only task remaining is for us to fill in the forms with your details, we would then submit them along with our plans to the relevant authorities and await approval. When the plans are passed you are free to commence with the building work. Building regulation drawings may be required on the larger jobs, which we can also produce for you.

If you do not have a builder in mind, we can forward our design drawings to a selection of suitable builders, to allow them to give you a full written quotation.

Below is a list of our recent Extensions & New Build projects that we have had successfully approved by Southampton Planning Department. Click here to view a list of our current projects on the Southampton City Council website.

For a free no obligation quotation call us today on 07870 219029.

Check List For Extensions and New Builds

There are 7 major stages passed during construction. At each stage the council will inspect the work to check it complies with their guidelines that are detailed below.

1. Commencement

Before you start any work you need to notify the council if you are proceeding under a Building Notice or a full plans approval submission (see our Building Regulations notes)

2. Digging Footings / Laying Foundations

  • Foundation concrete to be generally minimum 600mm wide and 150mm deep for cavity wall, and 450mm wide and 150mm deep for single skin walls.
  • Excavation depth in sandy or granular soils to be min 750mm deep.
  • Excavation depth in clay to be min 900mm and to invert of basements or any drains within 1m. -If poor ground is encountered this must be reported to Building Control or your structural engineer.
  • Excavation depth in clay will also be dependant on nearby trees.
  • Drains passing foundation trenches to be protected from foundation concrete and masonry
  • Existing foundations to be checked for suitability of any increased loadings ( i.e. Enlarged ‘break-throughs’ and 'build-overs')
  • Existing shallower foundations to be plugged with concrete at junction with new foundations to prevent the effects of undermining.

3. Damp-Proof Course Level

  • Height of DPC to be a minimum of 150mm above external ground level.
  • External wall insulation to achieve 0.3w/m2k. Typically 85mm cavities with full fill quilt cavity batts and thermal block.
  • DPC must be lapped to existing.
  • Check sufficient air bricks if timber floor i.e. 225x75mm air brick every 1m or 225mmx150mm every 2m.
  • New wall cavity to be maintained at junctions with existing or vertical DPC cut
  • Weak-mix concrete cavity fill required to 225mm below DPC level.
  • Cavity trays required over air brick sleeves or any other item bridging wall cavity.
  • Check that timber floor construction will comply if applicable. (150mm air void
  • between oversite level and underside of floor joists and joists to sit above DPC level).
  • Inner and outer DPCs to be same height.
  • Timber floor insulation to achieve 0.25 w/m2k i.e. 100mm of rigid foam insulation or 200mm of quilt on netting. Insulation not to obstruct air bricks.
  • Check size of break-through ( flush inner skin may need bonding) and calculations will be required if steel spans over 2.4m and if more than 2/3rd the width of wall is to be removed.
  • Check size of external corner returns and window/door sizes. Corner returns to be minimum 550mm and window/door widths not to exceed 2/3rd the overall wall width.
  • Sum of new doors, windows and roof lights in the extension not to exceed the sum of existing windows and doors lost as a result of the extension added to 25% of extension floor area.
  • Insulated DPC cavity closers to be used around all window and door openings.

4. Oversite – Laying Concrete Floor Slabs

  • Ensure minimum 100mm of concrete
  • Ensure 150mm levelled and compacted hardcore base.
  • DPM (damp-proof membrane) to be1200 gauge and laid on sand
  • blinding or 150mm thick polystyrene insulation.
  • Ensure DPM lapped to DPC all round.
  • If using rigid foam type insulations (Kingspan / Xtratherm etc) these
  • must be a minimum of 85mm thick and laid above the DPM)
  • Ensure old sub-floor vents maintained under slab with suitable ducting.
  • Ensure 25mm upstand insulation at door thresholds.
  • Drains within 300mm of hardcore below slab to be adequately protected i.e. compressible material to be placed over drain and floor slab to span across.

5. Drains

  • Check line and fall ( 1:40 ideal)
  • Ensure all drains can be rodded.
  • Check if manhole required i.e. when serving an internal soil branch or when changing direction or gradient.
  • Check bedding material ( 10mm pea gravel for plastic pipe)
  • Ensure gullies are trapped to foul and combined systems
  • Ensure approved connection to existing drains.
  • Check if separate system of drainage on site.

6. Pre-Plasterboard

As no statutory notice is required at this stage before covering any work up, you may wish to employ the services of your Structural Engineer or Architect to carry out an inspection should the scheme be complex or involve large span steelwork etc. It is imperative that these issues are correct before covering up, as the Council will only see the openings at completion stage and queries raised then will be more difficult to rectify.

  • Check roof, ceiling and floor timber sizes in accordance with Trada span tables.
  • Check all lintels, steelwork and associated piers and padstones are in accordance with your Structural Engineer’s details and calculations.
  • Check all hidden insulation is in place to manufacturers’ specification. Thermal insulation to roofs and ground floor joists with necessary ventilation gaps, sound insulation to upper floor joists and stud walls. All must be tight fitting to joists.
  • Check compartmentation to garage walls and ceiling. Minimum half hour.
  • Ensure half hour fire protection to all structural steelwork supporting walls and floors.
  • Check stair pitch max 42 degrees with max rise of 220mm, but min going of 220mm. ( As a general rule of compliance, the going must be approx 25mm greater than the rise)
  • Check no window or door openings to walls facing boundaries if within 1m.

7. Completion

All completion inspections will be carried out by SMBC Building Control, but the following advice should be noted in advance of giving notice.

  • Check window and door frames sealed to brick
  • Check safety glass in critical locations i.e. Windows below 800mm from floor, doors and adjacent panels within 1500mm of floor. ( must be marked to BS6206 or EN12150)
  • Restrictors to be fitted to opening lights where sills are below 800mm from floor surface. Restrictors must be able to be opened if window is needed for escape.
  • External balcony guarding to be minimum 1100mm high but max 1100mm if serving escape for a habitable room.
  • Check low e argon fill double glazing
  • Check ventilation/extract fans/discharge
  • Check visible plumbing and guttering and discharge.
  • Check escape windows to inner rooms and first floor rooms ( min 450mmx750mm clear opening.
  • Check mains powered, interlinked smoke detection in hall and landing areas on each storey.
  • Check 30 minute self-closing fire door between house and garage with 100mm minimum non-combustible step down.
  • Check roof insulation thickness. ( 275mm quilt in roof voids)
  • Check roof ventilation at soffits and abutments and at ridges if ‘room in the roof’ construction. ( Some breathable roofing felts will achieve the requirements but check manufacturers spec) Flat roof ventilation can be more complex and will require the submission of details.
  • Check roof tiles ( pitch/manufacturers spec) and flashings
  • Check electrical certification ( to be logged on line or copies to be provided)
  • Check Gas Safe Register certification. ( to be logged on line or copies to be provided)
  • Check headroom to new stairs. Min 2m vertically above pitch line required
  • Check stair pitch max 42 degrees with max rise of 220mm, but min going of 220mm. ( As a general rule of compliance, the going must be approx 25mm greater than the rise)
  • Check guardings to new stairs and landings whether internal or external. Min 900mm and max 1m above pitch line/floor and non climbable with gaps less than 100mm.
  • Check path level is 150mm below DPC.
  • Check roof structure if Building Notice. If no access a roof design must have been submitted for checking as requested by Building Control in previous correspondence. This also applies to floor joists.
  • Check self closing draught proof 30 minute fire door to internal garage access with min 100mm non-combustible step down.

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