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Help with estimated costs of extension / renovation in Cardiff


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Hi all,


Apologies for the long post, I think I posted this in the wrong section yesterday so will re-post here. I look forward to your comments:


I recently moved to a 6 bed 1960's detached house in Cardiff. This would have originally been a 4 bed house but has been extended (not so well) in a few parts of the house over the years. 


Below you will see the existing plans, and the proposed plans and here is a description of what changes we plan to make. I wonder if any of you could give ballpark figures on build costs for this, I know no one can be specific without visiting but perhaps experienced members may be able to give an idea? We have not yet submitted plans and do not have structural engineer calculations. I need to work out if this is even viable before submitting for planning. 


Ground floor changes:


1: Build front porch and extend garage out in line with new porch

2: Move WC into part of the garage

3: Build utility room in another part of the garage

4: Demolish 2 x small ground floor extensions - rear left and rear right of house

5: Build a ground floor 28sqm single story extension to house a open plan kitchen, dining, living room area (knock down wall between living room 2 and kitchen) 

6: Build a outside store at the rear of the house for BBQ, garden furniture, garden tools etc


First floor:


1: Block off existing stairs to above garage extension (very awkward, cut into room significantly and build off centre from main stairs) and create new access via new hallway through existing front/middle bedroo.

2: Existing front/middle bedroom also to become ensuite and walk in wardrobe for master bedroom and to house new stairs into loft (stacked over lower stairs)

3: Remove existing stairs into loft and create new family bathroom in this space (this is where original family bathroom was housed)

4: Existing family bathroom to become a bedroom (as it was originally), and steal a bit of space from master to allow more storage 

5: Above garage extension is currently awful, sloped roof at front means head height not great - we will add a front shed style dormer (many houses have been approved the same, or full 2nd story). New dormer will allow for ensuite to be housed in this area




Existing: Height height in loft about 1.9m currently and could be 2.2-2.3m. Loft extension (for unknown reasons) is made up of 2 small dormers with only about 10cm gap between them.

1: Remove existing dormers and rebuild with optimal head height / wider dormer, to house a bedroom, office and another bathroom

2: New access via new stairs




  • We will still have a 6 (or 5+ office) bedroom house - loose a bedroom on first floor (ensuite to master + walk in wardrobe) but gain a bedroom by converting existing family bathroom. 
  • We will have a more functional space downstairs for the family 
  • Loft and above-garage extension much more 'fluid' with the existing house 
  • Currently only 1 bedroom looks out onto the garden, the new plans we will have 4 with garden views




House is currently in a dated but liveable state with original parquet flooring downstairs which we plans to keep and restore

Will need either full rewire or update in areas

Walls pretty good so maybe some / all skimmed 

Boiler may need to be changed along with other plumbing 


Sorry for the incredibly long post, 


I look forward to any thoughts









Existing FF.png

Existing GF.png

Existing loft.png

Proposed plans.pdf

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For a little more info, attached is a ECA from a QS. This is significantly higher than we expected, and I know material and labour costs are high. We were hoping to get this done for 200K inc VAT. Based on the ECA, once we include all of the 'excluding' items, we are looking at a 300K project. 


For context, the house is in a nice part of Cardiff, end value once done up to a medium-high spec would be £775-825,000 subject to market conditions. Our purchase price was £540,000 earlier this year (managed to get 10% off asking price so we are comfortable that we managed to get a good deal at the time).

Screenshot 2023-06-11 at 09.42.07.png

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36 minutes ago, Dave Jones said:

be amazed if you get change from £350k


Many here including me do a lot of the work themselves thereby extending the duration but dropping the cost.

Thanks for your input Dave Jones. Any thought in %saving if self managing individual trades? It will require more of my time so will need to factor that in to the equation 

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id advise sending your plans to estimators.com who will give you real'ish prices so you have starting point. Project managing can save £££ for sure.


sometimes a step back before dropping a couple hundred K on a place for a quick sanity check.

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I would say the numbers do not add up on this project to make it feasible if those are the costs. However the costs do seem a bit too high based on how little extra extension you are adding to the property and the fact you are in Wales where build costs should be lower

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21 hours ago, Lofty718 said:

I would say the numbers do not add up on this project to make it feasible if those are the costs. However the costs do seem a bit too high based on how little extra extension you are adding to the property and the fact you are in Wales where build costs should be lower

With costs this high, I can’t see how any extension / renovation would be feasible in properties outside of very high value areas e.g London where the renovation:property value ratio would be much higher. I’ve got 2-3 builders to take a look so will see what sort of figures they come up with. 

thanks for your input 

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